Effects of Crumb Rubber-Modified Asphalt as a Pavement Layer in Railways: A Scoping Review

Abstract

Railway track performance and durability face growing challenges from higher speeds, heavier axle loads, and changing environmental conditions. Crumb rubber-modified asphalt (CRMA) offers a sustainable solution by repurposing waste tires into a durable material for railway trackbeds, improving both performance and environmental impact. Following PRISMA-ScR guidelines, this scoping review synthesizes an extensive body of global research on the structural, mechanical, and environmental benefits of CRMA in railway trackbeds. A systematic literature search was conducted across major academic databases, covering studies published over several decades. Selection criteria focused on CRMA applications in railway trackbeds, using keywords such as “crumb rubber-modified asphalt”, “railway track vibration”, and “sustainable railway materials.” After rigorous screening and eligibility assessment, the most relevant peer-reviewed studies were included, emphasizing mechanical performance, durability, and environmental impact. Key findings indicate that CRMA effectively reduces ground vibrations, enhances load distribution, and lowers long-term maintenance costs while promoting sustainable waste management through tire recycling. However, challenges such as optimal mix design, potential emissions, and long-term bonding stability require further investigation. Additionally, the review was limited to English-language studies, potentially omitting relevant non-English research, and some reports were inaccessible during retrieval. This review maps critical research gaps, identifies key areas for future optimization, and highlights CRMA’s potential to advance resilient and eco-friendly railway infrastructure.

1. Introduction

Railway track infrastructure is subjected to increasing operational demands due to higher speeds, heavier axle loads, and evolving environmental conditions. Traditional railway pavement materials, such as granular ballast and conventional asphalt layers, often struggle to meet modern railway systems’ durability and vibration-damping requirements. In response, crumb rubber-modified asphalt (CRMA) has gained significant attention as a sustainable, high-performance alternative. CRMA, produced by incorporating waste tire rubber into asphalt mixtures, offers enhanced flexibility, improved fatigue resistance, and superior vibration attenuation, making it an attractive choice for railway applications [1,2,3,4,5,6,7,8,9,10].

Recent studies indicate that CRMA effectively improves the fatigue life of railway pavements, particularly in high-speed railway systems, where conventional asphalt layers deteriorate under cyclic loading [6,7,8,9,10,11,12,13,14,15]. Additionally, railway infrastructure faces challenges due to increased axle loads, traffic volumes, and train speeds, leading to the growing use of asphalt stabilized ballast (ASB), asphalt underlayment (AUL), and asphalt slab track bed (ASTB) in maintenance and repair applications [16,17,18,19]. Rubber-modified asphalt (RMA) presents a promising solution for high-speed rail, providing enhanced stiffness and damping characteristics compared to traditional materials, significantly reducing vibrations [20]. Papers [21,22,23,24,25,26] demonstrated that combining CRMA with expanded polystyrene (EPS) barriers led to a substantial reduction in ground vibrations, making it a viable option for sustainable railway infrastructure.

Furthermore, innovative applications of waste tire rubber in railway substructures are being explored to reduce energy demands and improve the life cycle performance of track systems [27]. The addition of rubber to asphalt enhances its dynamic performance, fatigue resistance, and crack resistance under heavy cyclic loads, aligning with sustainability objectives [28,29,30,31]. Killas asphalt concrete layers, for instance, are being developed as semi-rigid ballast layers to enhance load distribution, vibration reduction, and durability in high-speed railway tracks [32]. Moreover, crumb rubber porous asphalt mixtures are being investigated for their potential to mitigate traffic noise pollution and contribute to urban and environmental sustainability [33].

To address limitations associated with traditional rubberized asphalt, such as excessive viscosity and environmental concerns, desulfurized rubber asphalt (DRA) has been introduced as a more sustainable alternative with improved road performance characteristics [34]. Track geometry maintenance and substructure resilience can also be enhanced by incorporating rubberized materials into maintenance techniques, such as the two-step stoneblowing process combined with recycled rubber particle treatments [35,36,37,38]. Research on dry asphalt rubber concrete (DARC) has further confirmed the benefits of incorporating recycled crumb rubber into sub-ballast layers, demonstrating superior flexibility and vibration mitigation [39]. Additionally, rubber-modified asphalt concrete exhibits excellent crack resistance and fracture behavior, positioning it as a cost-effective substitute for polyurethane-based trackbeds [40].

The versatility of rubber-modified materials extends beyond sub-ballast applications. Studies indicate that rubber-modified RacFlax pavements improve physical properties under variable temperature conditions, while rubberized sub-ballast layers in high-speed rail networks enhance mechanical efficiency and anti-aging characteristics [41,42]. Furthermore, the integration of thermoplastic polyurethane (TPU) blended with waste rubber powder (WRP) has been proposed as a waterproof sealing layer to enhance moisture resistance and mechanical stability under harsh environmental conditions [43]. Waste tire rubber (WTR) is also being utilized in ballasted railway tracks, particularly in ballast mixtures, under-sleeper pads (USPs), and under-ballast mats (UBMs), demonstrating its potential to improve track resilience, vibration mitigation, and sustainability [44].

Despite the numerous advantages of CRMA, some challenges remain. The optimal mix design, potential emissions, and long-term bonding stability require further investigation to maximize its effectiveness in railway infrastructure applications [45]. For instance, CRMA’s performance in high-temperature environments and its susceptibility to long-term aging remain key research areas [46]. Given the increasing adoption of CRMA in railway infrastructure, a comprehensive scoping review is essential to consolidate existing knowledge, identify research gaps, and highlight future directions. This review follows the PRISMA-ScR framework to map current evidence on CRMA applications in railway trackbeds systematically. The study selection criteria include research published between 1991 and 2025 (see the related graph in Figure 1), focusing on mechanical performance, durability, and environmental impact. This review aims to provide valuable insights for optimizing CRMA applications and promoting sustainable railway infrastructure development by synthesizing available research.

2. Materials and Methods

2.1. Background

Due to its superior mechanical properties, CRMA is increasingly recognized as an eco-friendly and durable material for railway and pavement applications. CRMA incorporates recovered crumb rubber from end-of-life tires, enhancing flexibility, reducing cracking, and improving performance under dynamic loads, making it suitable for railway infrastructure [47,48]. The three-step grinding process for producing crumb rubber results in granules of various sizes that can be mixed with asphalt [47]. CRMA contributes to sustainability by repurposing scrap tires, reducing landfill waste, and lowering the carbon footprint of construction projects—up to 1000 tires can be recycled per kilometer of roadway [48]. However, widespread adoption is hindered by variations in mixing and compaction temperatures across different climatic regions and long-term property alterations in the mix [47,48].

Key parameters for optimizing CRMA formulations include particle size, rubber content, and environmental impacts such as leachates containing organic chemicals and metals [47,48,49]. CRMA exhibits greater stiffness and damping properties than traditional materials, making it an effective solution for railway vibration control compared to conventional ballast and asphalt [1,6,50,51,52,53]. Initially developed for railway trackbeds in the late 1980s, its formulation was later modified to incorporate recycled rubber, improving stiffness, vibration damping, and durability [6,11,54,55,56]. With the rapid expansion of high-speed railways, concerns about ground vibration and noise pollution have intensified, particularly in urban areas. Studies indicate that low-frequency vibrations can be significantly reduced when CRMA is used in combination with EPS barriers [21]. CRMA also enhances durability and reduces maintenance costs compared to compacted soil alternatives [20]. However, challenges remain, including balancing elasticity and stiffness in track structures [57,58,59].

Recent advancements include the development of asphalt concrete layers as an alternative to stone ballast systems to enhance structural performance and energy efficiency [32]. CRMA and other recycled materials demonstrate lower embodied energy and greenhouse gas emissions; yet, they also exhibit variability in composition and durability in practical applications [27,28]. Additionally, rubber-modified asphalt improves fatigue and crack resistance, but further studies are needed to assess the impact of rubber particle content on fracture behavior [40]. The dry process, which involves mixing rubber granules with aggregates before adding asphalt binder, is widely preferred due to its simplicity and compatibility with conventional construction methods [41]. With increasing axle loads and higher train speeds, CRMA adoption is growing in high-speed railway networks, similar to Spain’s PeT 2020 plan [42,60]. Moreover, conventional waterproofing strategies deteriorate under heavy loads and adverse weather conditions, leading to the exploration of TPU/WRP composites for improved stiffness, cost efficiency, and waterproofing [43]. Crumb rubber in porous asphalt mixtures will absorb traffic noise; however, it must be optimized for durability [33]. The desulfurization process enhances rubber compatibility in asphalt, but variations in performance across methods require further systematic evaluation [34]. Waste tire rubber (WTR) strengthens ballast resistance to deformation; yet, its long-term performance remains uncertain [44]. The use of rubber has evolved over the past century from ballast stabilization to vibration damping and load distribution; yet, long-term degradation continues to be a challenge with asphalt-based materials (ABM) [16]. CRMA and rubber-modified concrete, which offer flexibility, heat resistance, and improved fatigue life, are also used to modernize railway structures [45,46]. Figure 2 shows the process of converting waste tires into crumb rubber through grinding, then integration into asphalt mixtures for use in railway and road construction.

The evaluation criteria in this study were selected based on a thorough review of previous research directly related to the topic. Furthermore, an extensive review of indirectly related studies was conducted to ensure that the selected criteria align with broader research perspectives. This approach helps establish a meaningful intersection with relevant studies and highlights the significance of these criteria in the context of this review article.

2.2. Methodology

2.2.1. Research Method of This Review

In this article, after the purposeful selection of articles from various years, the process of reviewing the articles has been carried out to provide a comprehensive and precise analysis, as shown in Figure 3. The process starts with formulating and defining key topics, followed by categorizing and analyzing articles based on specific criteria. Finally, the results are summarized and clearly presented in Section 5.

This scoping review included studies investigating the application of CRMA in railway infrastructure. Inclusion criteria encompassed studies published in English that specifically addressed the mechanical performance, durability, and environmental impacts of CRMA in railway pavement. Studies focusing on non-railway applications of CRMA or lacking experimental or observational data were excluded. Studies were grouped based on application type (e.g., ballast, sub-ballast, or asphalt layers) and evaluation methods (laboratory, numerical, or field-based). Relevant studies were identified by searching Scopus, Web of Science, and Google Scholar. Additionally, reference lists of pertinent articles and websites of transportation and railway organizations were reviewed.

The search strategy employed keywords such as “crumb rubber-modified asphalt”, “railway track vibration”, “rubberized asphalt”, “sustainable railway materials”, and “waste tire recycling in railways.” Searches in Scopus and Web of Science were limited to English-language articles published between 1991 and 2025. Similar temporal and linguistic filters were applied in Google Scholar. Two reviewers independently screened the titles and abstracts of identified articles. Studies that clearly did not meet the inclusion criteria were excluded. Full texts of potentially relevant articles were retrieved and assessed for eligibility. Discrepancies between reviewers were resolved through discussion and consensus. No automated tools were used in the screening process. Data were extracted from each included study by one reviewer and verified for accuracy by a second reviewer. Extracted data included authors, publication year, methodology, key findings, and study limitations. In cases of missing or unclear data, attempts were made to contact the study authors. No automated tools were utilized for data extraction. The primary outcomes of interest were mechanical performance (e.g., fatigue resistance, stiffness, and vibration damping capacity), durability (e.g., resistance to cracking and aging), and environmental impacts (e.g., greenhouse gas emission reduction and tire recycling efficiency). All results aligned with these domains were extracted. Variables such as participant characteristics or funding sources were not collected, as they were deemed irrelevant to the review’s objectives. Studies were grouped for synthesis based on CRMA application in railway infrastructure (e.g., ballast, sub-ballast, or asphalt layers) and evaluation methods (laboratory, numerical, or field-based). Data were synthesized qualitatively, with key findings summarized in tables and figures (e.g., Figure 3: Flowchart of the article review process; Figure 4: Examples of recycled tire crumb rubber applications in railway pavement layers). As this is a scoping review without meta-analysis, statistical methods for assessing heterogeneity or conducting sensitivity analyses were not employed. Given the scoping nature of this review, formal assessments of study risk of bias, reporting bias, or evidence certainty (e.g., GRADE) were not conducted. However, the methodological quality of each study was qualitatively discussed in the results section, and the limitations of the included studies and knowledge gaps were addressed in the discussion.

The study selection process was conducted following the PRISMA 2020 guidelines. The details of the screening and selection process are illustrated in Figure 4.

2.2.2. Review of Common Methods

In scientific research, various methodologies—such as numerical simulations, laboratory experiments, field tests, and computational modeling—are commonly used to evaluate the performance of crumb rubber-modified asphalt (CRMA). Previous studies have employed these approaches and may also be considered for future investigations. Figure 4 provides significant examples of CRMA applications in railway pavement layers.

Numerical simulations and laboratory tests have extensively assessed CRMA’s dynamic characteristics under different temperature and pressure conditions, proving its effectiveness in vibration reduction and railway track performance enhancement [1,6]. These studies also provide valuable material specifications and construction guidelines. VECD models, creep compliance tests, and cyclic fatigue tests have shown that CRMA mixtures with 2% air void content achieve 7.2 times longer fatigue life under low-stress conditions compared to unmodified asphalt [11,61,62,63]. Other investigations have applied dynamic modulus, resonant column, and finite element modeling (FEM) techniques to evaluate rubber-modified asphalt’s response to high-speed railway loads [16,20,32].

Further research confirmed CRMA’s vibration attenuation capabilities through centrifuge model tests and FFT analysis in combination with EPS barriers [21,64,65,66]. Full-scale laboratory models have also quantified improvements in track resilience, lateral resistance, and settlement recovery when incorporating rubber composite sleepers and tire-derived aggregate (TDA) into ballasted track designs [35,39,57]. CRMA’s application in railway engineering is further optimized through advanced characterization techniques, including Marshall stability tests, indirect tensile strength measurements, and scanning electron microscopy (SEM) analysis, which refine rubberized asphalt mix designs for cost-efficient production [28,33,39,45]. Extended finite element simulations, stress analysis, and 3D dynamic load modeling have facilitated the comprehensive evaluation of CRMA’s thermal behavior, fatigue life, and environmental impact [27,42,43]

ASTM Standard tests, such as penetration, softening point, viscosity, and ductility tests, were developed to evaluate rubber-modified binders’ physical and mechanical properties. All of these tests yielded positive results, confirming compliance with engineering standards [45]. Together, these combined approaches highlight the potential of CRMA to enhance railway durability, reduce maintenance costs, and address sustainability challenges through innovative use of recycled materials [1,6,35,42,43,45,67].

The methodologies used to evaluate CRMA in railway infrastructure encompass a variety of experimental, numerical, and analytical techniques.

Table 1. Overview of methodological approaches for evaluating CRMA in railway infrastructure.

Category of Methodology	Objective/Description	Techniques/Tools Used
Laboratory Dynamic Tests	Evaluate the dynamic properties of CRMA under varying temperature and pressure conditions and assess vibration reduction in the railway track.	Laboratory tests, numerical simulations, dynamic modulus tests, fatigue tests [1,6].
Fatigue Evaluation and VECD Modeling	Investigate the fatigue properties of CRMA using viscoelastic continuum damage (VECD) theory with creep, monotonic loading, and cyclic fatigue tests to enhance fatigue life.	Creep tests, monotonic loading, cyclic fatigue tests, VECD modeling [11].
Performance Evaluation of ABM in Railways	Assess the overall performance of ABM systems through a combination of laboratory tests, field evaluations, and numerical simulations.	Dynamic modulus tests, fatigue tests, large-scale tests, finite element analysis (FEM) [16].
Resonant Column Tests for RMA	Measure the stiffness and damping ratio of RMA to determine its vibrational performance and resistance to dynamic loads.	Resonant column tests [20].
Centrifuge Model Tests for Vibration	Simulate the effects of CRMA and EPS barriers on reducing ground vibrations caused by high-speed trains using frequency analysis.	Centrifuge model tests, multiple ball-dropping system, FFT analysis [21].
Integration of Sustainable Materials (TDA)	Evaluate the use of tire-derived aggregate (TDA) as a sustainable substitute in railway substructures by investigating enhanced durability, reduced ballast degradation, and optimized lateral resistance.	Controlled dosage determination of TDA, laboratory evaluations [67].
Vibration Tests for Rubber Composite Sleepers	Investigate the vibration reduction characteristics of rubber composite sleepers compared to conventional concrete sleepers.	Full-scale laboratory model, drop hammer testing, time and frequency domain analysis [57].
Energy Assessment and Analysis of Recycled Materials	Conduct a comprehensive assessment of the physical, mechanical, and chemical properties of recycled materials and evaluate energy consumption from extraction to installation.	Laboratory tests, energy modeling, life cycle energy assessment (LCEA) framework, statistical analysis [27].
Characterization of Rubberized Asphalt	Evaluate the mechanical and microstructural performance of rubberized asphalt using Marshall tests, indirect tensile strength tests, dynamic modulus, fatigue tests, and microscopic analysis.	Marshall tests, indirect tensile strength tests, dynamic modulus tests, fatigue tests, SEM, spectroscopy, LCEA framework [28].
Evaluation of Asphalt Support Layer for Rail	Assess the asphalt layer as a rail support in terms of stiffness, dynamic modulus, and damping characteristics under dynamic loading.	Static and dynamic load tests, fatigue and impact tests, finite element modeling (FEM) [32].
Testing of Porous Asphalt Mixtures	Evaluate the mechanical properties, durability, and noise reduction performance of porous asphalt mixtures containing crumb rubber.	Porosity tests, permeability tests, sound absorption tests using standard methods [33].
Full-Scale Laboratory Simulation of Ballasted Railway Sections	Simulate the performance of ballasted railway sections with varying settlement levels and evaluate stiffness, rail oscillations, and settlement recovery.	Construction of a full-scale laboratory section, application of dynamic loads (5 Hz), and measurements using LVDTs [35].
Full-Scale Laboratory Simulation of Sub-Ballast Sections	Simulate sub-ballast and ballast layers using Marshall mix design, four-point bending tests, and 2D lumped mass modeling to assess sub-ballast performance.	Four-point bending tests (4PB-PR), 2D lumped mass modeling, dynamic testing protocol [39].
Numerical Modeling for CRMB Sub-Ballast	Assess the structural behavior of CRMB sub-ballast—including load distribution, settlement, fatigue life, and thermal effects under varying conditions.	Finite element modeling in ANSYS, (Ansys Inc., Canonsburg, Pennsylvania, United States of America; ANSYS 13.0) application of the dynamic axle load criterion from the Eisenmann model [42].
Mechanical Evaluation of TPU/WRP	Evaluate the mechanical performance and water resistance of the TPU/WRP composite as a waterproof and load-distributing layer in the railway substructure.	Compressive tests, permeability tests, FEM modeling (3D and 2D), dynamic elastic modulus tests [43].
Laboratory/ Standard Tests for Asphalt Properties	Determine the physical properties of asphalt—including hardness, softening point, flash point, ductility, viscosity, and specific gravity—to evaluate changes from crumb rubber addition.	Utilization of standard ASTM tests (D5, D36, D92, D113, D4402, D70) [45].

summarizes the key methodological approaches, outlining their objectives and the tools employed to assess CRMA’s mechanical properties, durability, and vibration mitigation performance.

2.3. Material Analysis

Enhanced asphalt and railway materials have been analyzed using advanced chemical, mechanical, and rheological techniques. GC-MS analysis identified key additives in crumb rubber granules (CRG) using full-scan and PAH-SIM techniques [49,68,69,70]. Comparative studies on desulfurized rubber powder and conventional rubberized powder examined their effects on asphalt performance using microwave, ultrasound, and biological desulfurization methods [34]. Two types of sleepers, concrete (for 25-ton axle loads) and rubber composite (recycled plastic-bitumen) were analyzed, with the latter demonstrating improved sustainability and reduced maintenance requirements [57]. Rubber particles (0.2–4.75 mm) were incorporated into various asphalt mixtures, optimized according to Superpave PG criteria [40,41,45,71,72,73]. Sub-ballast systems utilized conventional materials, such as crushed gravel and sand, enhanced with crumb rubber-modified bitumen (CRMB) to improve load-bearing capacity for railway applications [42]. The study also investigated TPU/WRP composites containing waste rubber powder (25–35%) and enhanced with defoaming agents [43]. Waste tire rubber (WTR) was categorized by particle size (shreds, chips, crumb, and powder), with smaller fractions applied in rail pads, sleepers, and asphalt, while larger fractions were used in under-ballast mats [44]. Additionally, the incorporation of silica fume (5–10%) was found to enhance the strength of rubber-sand concrete [46]. Various analytical techniques and material characterization methods have been employed to evaluate the structural and mechanical properties of CRMA components in railway infrastructure.

Table 2. Overview of material analysis for CRMA components and railway infrastructure elements.

Component	Analysis/Methodology	Key Details
Crumb Rubber Granules (CRG)	Comprehensive GC-MS analysis using three approaches.	Non-target full-scan for all GC-amenable additives; SIM for PAHs; SIM for benzothiazole compounds; equipment: Agilent 7890A with Agilent 5975C MSD (Agilent Technologies, Inc., Santa Clara, California, United States of America), EI source; detailed conditions in Supplementary Info [49].
Sleepers	Material characterization of railway sleepers.	Standard Concrete Sleeper Type II (for freight/passenger, 25-ton axle load, 120 km/h) and Rubber Composite Sleeper Type I (made from recycled plastic, bitumen, fillers; low maintenance, sustainable) [57].
Asphalt Binder and Rubber Powders	Binder formulation and rubber powder production.	Uses Dongming 70# base asphalt with two rubber powders (desulfurized and conventional); twin-screw shearing desulfurization method ensuring simpler production, lower cost, and minimal environmental impact [34].
Asphalt Concrete Mixtures	Preparation of asphalt concrete with rubber particles as a substitute for polyurethane-based trackbeds.	Two binders used: epoxy-modified and rubber-modified; recycled tire rubber particles sized 2.36–4.75 mm; four FAM mixtures with varied rubber contents (9% and 11% for epoxy–FAM, 3% and 6% for rubber–FAM) [40].
Aggregate Gradations	Evaluation of different aggregate gradations in asphalt mixtures.	Comparison of dense-graded asphalt concrete (max 22.4 mm) vs. gap-graded stone mastic asphalt (max 31.5 mm); rubber content of 1.5–3% by weight (particle sizes 0.2–4 mm); binder per Superpave PG criteria [41].
Sub-Ballast Comparison	Comparative analysis of conventional vs. CRMB (crumb rubber-modified bitumen) sub-ballast.	Conventional sub-ballast: crushed gravel and well-graded sand; CRMB: standard bitumen modified with finely ground recycled tire rubber to ensure compatibility with railway trackbed requirements [42].
TPU/WRP Composite	Composite formulation and characterization.	Consists of a TPU matrix (from polyol and diisocyanate), waste rubber powder (0.16 mm from end-of-life tires), and a PU defoaming agent; formulation optimized by varying PU-to-rubber ratio (25–35% WRP) [43].
Waste Tire Rubber (WTR) Categorization	Classification based on particle size.	Categorized into shreds (50–300 mm), chips (10–50 mm), crumb rubber (0.425–12 mm), and powder (<1 mm); selection depends on the application (e.g., under-ballast mats, rail pads, rubberized sleepers) [44].
Bitumen Modification with Crumb Rubber	Testing the effects of adding crumb rubber to bitumen.	Uses 60/70 penetration-grade bitumen modified with 1–4% crumb rubber (particle size: 0.3 mm) to assess impacts on asphalt’s physical properties [45].
Crumb Rubber as Fine Aggregate Replacement	Partial replacement of fine aggregates with crumb rubber and silica fume addition.	Uses crumb rubber particles sized 75 µm, 180 µm, and 400 µm; silica fume added (0%, 5%, and 10%) to compensate for strength loss; various mix designs evaluated [46].

provides an overview of the key material analysis approaches, detailing the methodologies used for different CRMA components and railway elements.

2.4. Data Processing and Quality Control

Data processing techniques ensure methodological integrity and reproducibility. Chromatogram peaks are analyzed using deconvolution algorithms with the NIST 2017 library [47], while physical and rheological parameters are assessed through penetration, softening point, ductility, and viscosity tests [34,74,75,76,77]. Viscosity-temperature curves, MSCR tests, and PG classification further refine asphalt binder properties. The superpave gyratory compactor (SGC) simulates field compaction conditions, while Marshall stability, indirect tensile strength (ITS), and cyclic fatigue tests assess durability [41]. FEM and DEM simulations model load distribution, settlement, and long-term performance [44]. Ensuring the quality and reliability of CRMA and waste tire rubber (WTR) applications in railway infrastructure requires robust data processing and standardized evaluation techniques.

Table 3. Data processing and quality control techniques for CRMA and WTR.

Aspect	Method/Technique and Tests	Key Details
Chromatogram Data Processing	Deconvolution algorithms; compound identification via matching with the NIST 2017 library.	Peaks are filtered to include compounds present in at least 3 of 6 replicates with >90% match; biogenic compounds are excluded to refine the dataset [49].
Standardized Laboratory Testing Protocols	Basic pavement performance tests (penetration, softening point, ductility, viscosity); rheological tests (dynamic shear, bending beam); aging and storage stability tests; gas emissions measurement.	Comprehensive testing to determine the mechanical and thermal behavior of DRA using viscosity-temperature curve analysis, MSCR tests, and performance grading (PG) classification [34].
Mix Design and Field Simulation	Use of superpave gyratory compactor (SGC) for mix design; evaluation via Marshall stability and flow tests, indirect tensile strength (ITS), thermal conductivity, and fatigue tests; FEM modeling for thermal performance.	Target air void content is set at 3% for optimal performance; a finite element model developed to predict the thermal behavior of sub-ballast layers under varying environmental conditions [41].
Performance Assessment of WTR-Modified Components	Laboratory and field tests; finite element method (FEM) and discrete element method (DEM) simulations.	Assess mechanical performance, vibration damping, and environmental durability; simulations used to predict load distribution, stress transfer, and settlement behavior [44].

summarizes the key methodologies used for data processing, laboratory testing, mix design optimization, and performance assessment of CRMA-based materials.

Scientific research is conducted to improve real-world conditions and enhance predictive accuracy. The findings can be practically implemented, observed, and evaluated by performing structured and continuous investigations. If necessary, future modifications can be made to optimize outcomes and ensure their effectiveness in diverse applications.

2.5. Analytical Techniques

Advanced analytical techniques evaluate CRMA performance at multiple scales. ICP-MS measures metal content under strict quality control standards [32,47,49,78,79]. Vibration performance is assessed through time- and frequency-domain analysis, quantifying reductions in vibrations at track, ballast, and subgrade levels [57]. Rheological tests such as dynamic shear rheometer (DSR), bending beam rheometer (BBR), and multiple stress creep recovery (MSCR) evaluate resistance to rutting and fatigue [34]. Fracture resistance is studied using semi-circular bending (SCB) tests, acoustic emission (AE) sensors, and X-ray computed tomography (CT) scanning [40,80,81,82,83]. Mechanical performance is validated through triaxial compression, resilient modulus, and ballast box tests, supported by FEM and DEM simulations [44]. Various advanced analytical techniques have been employed to assess the chemical, mechanical, and rheological properties of CRMA and WTR-modified components.

Table 4. Overview of analytical techniques for CRMA and WTR-modified components.

Analytical Technique	Method/Tools Used	Key Details
Chemical Quantification and ICP-MS Analysis	Six-level calibration curve normalized to internal standards; ICP-MS for metal content in leachates.	Comprehensive assessment of chemical characteristics of crumb rubber via deconvolution of chromatogram peaks and stringent filtering criteria [49].
Quality Control Framework in Chemical Analysis	Systematic approach addressing data gaps with rigorous quality control measures.	Establishes a robust framework for analyzing chemical properties of crumb rubber in modified asphalt, ensuring reliable and accurate results [47].
Vibration Performance Analysis	Time-domain analysis (peak acceleration, vibrational energy, RMS); frequency-domain analysis using Fourier Transform to obtain power spectral density (PSD); transfer loss calculation.	Evaluates dynamic responses of track components (e.g., sleepers) to assess vibration reduction in sleepers, ballast, and ground through both time and frequency domain analyses [57].
Rheological and Environmental Impact Assessment	Dynamic shear rheometry (DSR), bending beam. Rheometry (BBR), gas chromatography, multiple stress creep recovery (MSCR) test.	Measures rutting resistance and assesses rheological behavior of DRA; advanced analytical tools provide precise evaluation of mechanical and environmental performance characteristics [34].
Fracture Behavior and Crack Propagation Analysis	Specimen preparation (controlled mixing, cylindrical and semi-circular test pieces); semi-circular bending (SCB) test at multiple temperatures; acoustic emission (AE) analysis; X-ray CT scanning.	Investigates fracture mechanics by recording crack initiation and propagation; AE sensors coupled with CT scanning offer insights into internal crack development and controlled crack propagation mechanisms [40].
Mechanical Performance and Numerical Modeling	Triaxial compression tests; resilient modulus testing; railway ballast box tests; finite element method (FEM) and discrete element method (DEM) simulations.	Assesses shear strength, deformation characteristics, and real-world loading conditions for WTR-modified components; numerical models (FEM/DEM) simulate load distribution, stress transfer, and settlement behavior under load [44].

outlines the key methods used in chemical quantification, vibration performance evaluation, fracture behavior analysis, and mechanical modeling, providing insights into their effectiveness in railway applications.

These methodologies collectively validate CRMA’s mechanical, structural, and environmental advantages, highlighting its potential for improving railway infrastructure durability, reducing maintenance costs, and advancing sustainability goals.

The analytical methods employed in this study remain relevant and applicable due to their adaptability to evolving research needs. Their effectiveness in addressing critical challenges ensures their continued significance in both theoretical and practical applications.

3. Results

3.1. Study Selection

The initial search across Scopus, Web of Science, and Google Scholar identified 1253 records. After removing 102 duplicates, 1002 records were screened based on their titles and abstracts. Of these, 701 records were excluded because they did not meet the inclusion criteria. Full texts of 301 reports were sought for retrieval, but 49 reports could not be accessed. Ultimately, 252 reports were assessed for eligibility, and 182 reports were excluded for the following reasons: not exclusively focusing on railway pavement layers (21 reports), using different materials (46 reports), lacking a focus on effects (52 reports), and being conducted in an inappropriate test environment (63 reports). As a result, 70 new studies were included in this scoping review. The selection process is illustrated in Figure 5 (PRISMA flow diagram).

Some studies initially appeared relevant but were excluded because they solely focused on CRMA applications in highway pavements without addressing railway infrastructure, thus failing to meet the inclusion criterion of focusing on railway infrastructure.

3.2. Effects of Crumb Rubber-Modified Asphalt

CRMA is an innovative material for railway infrastructure, addressing the limitations of traditional composites. It significantly enhances crack resistance, fatigue durability, and overall track lifespan, leading to reduced maintenance needs [16,28]. Environmentally, CRMA promotes circular economy practices by recycling waste tires, lowering energy consumption, and reducing greenhouse gas (GHG) emissions [27,33,67,84,85,86].

Key benefits of CRMA in railway applications include improved load distribution, enhanced vibration damping (40–127% higher at low temperatures), and over 30% reduction in ballast stress—critical factors for high-speed rail operations [32,35,39]. Additionally, CRMA contributes to noise reduction, improved waterproofing, and better subgrade protection while stabilizing sleeper transitions to enhance track performance [33,35,39,60].

Despite its advantages, challenges remain, particularly regarding volatile organic compound (VOC) emissions and long-term bonding durability, which necessitate further research [16,33,87,88,89]. Overall, CRMA strengthens railway sustainability, operational efficiency, and resilience, making it a promising material for future railway infrastructure advancements [27,28,60,67].

Table 5. Effects of crumb rubber-modified asphalt (CRMA) on railway infrastructure.

Aspect	Key Findings and Benefits	Implications for Railway Infrastructure
Mechanical Performance and Durability	CRMA exhibits improved resistance to cracking, temperature fluctuations, and fatigue. Studies report enhanced flexibility, increased fatigue life, reduced cracking, and lower maintenance needs.	Leads to longer service life, improved structural integrity, and reduced frequency of track repairs and maintenance [16,27,28].
Environmental Sustainability	The incorporation of recycled tire rubber reduces the demand for virgin materials, lowers embodied energy and greenhouse gas emissions, and contributes to waste reduction and a circular economy.	Enhances the sustainability of railway substructures by lowering the environmental footprint and reducing life cycle costs associated with material production and maintenance [27,28,67].
Vibration Damping and Load Distribution	Incorporating asphalt layers (including DARC) improves load distribution and vibration damping. Although DARC shows a lower stiffness modulus (about half that of conventional mixtures), it achieves significantly higher damping ratios, reducing dynamic stresses and rail oscillations.	Results in better track geometry maintenance, improved ride quality, and extended service life, particularly important for high-speed rail operations [32,35,39].
Noise Reduction	Adding crumb rubber to porous asphalt mixtures significantly improves sound absorption due to increased porosity and elasticity, thereby enhancing the damping of traffic-induced vibrations.	Contributes to a quieter rail environment and improved quality of life in urban areas by reducing noise pollution from rail operations [33].
Additional Infrastructure Benefits	Asphalt trackbeds offer improved load distribution, moisture protection, and drainage while reducing maintenance frequency and associated costs.	Provides enhanced protection of the subgrade, ensures optimal moisture conditions, and reduces overall maintenance expenditures [60].
Potential Challenges	Some challenges include the potential release of volatile organic compounds (VOCs) and long-term bonding issues associated with CRMA, which require further investigation and optimization.	Highlights the need for ongoing research and mitigation strategies to ensure health, environmental safety, and long-term performance reliability in railway applications [16].

outlines the primary effects of CRMA on railway infrastructure, focusing on key aspects such as mechanical performance, durability, environmental sustainability, vibration damping, noise reduction, and additional structural benefits. CRMA improves track longevity, enhances load distribution, and mitigates dynamic stresses, making it particularly valuable for high-speed rail systems. Furthermore, its ability to lower maintenance frequency and life cycle costs aligns with modern sustainable railway practices. While CRMA presents numerous advantages, challenges such as VOC emissions and long-term bonding stability highlight the need for continued research and optimization.

3.2.1. Environmental Impact

CRMA and recycled rubber contribute to sustainability by reducing waste and lowering carbon footprints. With approximately 1.5 billion tires discarded annually, CRMA applications such as rubber-modified asphalt and composite sleepers help divert significant amounts of waste from landfills [1,6,11,20,40,41,42,43,46,57,90,91]. These materials also contribute to lower emissions and reduced energy consumption; for example, the production of desulfurized rubber asphalt (DRA) results in 50% fewer hydrocarbon emissions [34]. Additionally, CRMA extends infrastructure lifespan, enhancing long-term performance and sustainability [41,45,48].

In railway applications, CRMA improves drainage efficiency, reduces subgrade pressure, and lowers track vibrations by up to 76% [57,92,93,94]. The use of rubberized sub-ballast has been shown to recycle approximately 1500 tires per kilometer of track, further supporting circular economy initiatives [41,42]. Other benefits include traffic noise reduction [44], decreased CO₂ emissions [45], and lower environmental pollution levels [90]. Additionally, TPU/WRP composites and rubberized waterproofing solutions enhance resource efficiency and climate adaptability, making railway infrastructures more resilient to environmental challenges [40,43,45]. These innovations align with global sustainability objectives, promoting greener construction practices in the transportation sector [1,6,20,21,46,90].

Table 6. Environmental impact of crumb rubber-modified asphalt (CRMA) in railway infrastructure.

Environmental Aspect	Key Findings and Benefits	Implications for Railway Infrastructure
Waste Tire Recycling	CRMA repurposes non-biodegradable waste, reducing landfill accumulation and pollution. Over 1.5 billion tires are generated annually, making recycling crucial for sustainability.	Reduces environmental hazards from tire stockpiling and burning, contributing to a circular economy [1,45,62,91].
Carbon Footprint Reduction	Using CRMA lowers CO2 emissions by reducing the demand for virgin materials and requiring less energy in production.	Leads to lower embodied energy in railway substructures and a reduced overall carbon footprint [6,20,44,48].
Noise and Vibration Reduction	Rubberized track components, including CRMA, lower noise pollution by 1–2 dB and mitigate ground vibration transmission.	Enhances the quality of life in urban areas and reduces environmental disturbances caused by railway operations [21,44,57].
Reduced Harmful Emissions	CRMA produces fewer volatile organic compounds (VOCs) and gas emissions than traditional rubberized asphalt (RRA).	Minimizes air pollution during material production and extends sustainability benefits of railway infrastructure [34].
Resource Efficiency and Circular Economy	The use of recycled rubber in CRMA and rubber composite sleepers reduces reliance on virgin materials, lowering the environmental footprint of railway infrastructure.	Supports circular economy initiatives by promoting waste reduction, resource efficiency, and sustainable construction practices [40,43,57,67].
Energy Savings	CRMA production requires lower mixing temperatures, reducing energy consumption compared to conventional asphalt.	Leads to lower energy demand in railway material production and maintenance [27,45].
Climate Adaptability	CRMA enhances asphalt durability in extreme climates, making it suitable for high-temperature and cold-weather railway operations.	Provides long-term resilience in railway tracks, reducing maintenance needs in harsh environmental conditions [45].

summarizes the key environmental benefits of CRMA in railway infrastructure. These benefits include waste tire recycling, carbon footprint reduction, noise and vibration mitigation, lower emissions, improved resource efficiency, and enhanced climate adaptability. CRMA contributes to a circular economy by repurposing waste materials and reducing reliance on virgin resources. Additionally, its energy-saving properties and durability in extreme climates enhance railway sustainability while lowering maintenance costs. The findings in this table highlight CRMA’s role in advancing eco-friendly and long-lasting railway infrastructure solutions.

3.2.2. Performance Enhancements

CRMA significantly enhances pavement infrastructure by improving resistance to temperature variations, rutting, thermal cracking, fatigue, and adhesion [47,95]. In railway applications, it increases track durability by offering superior vibration damping (61.8% reduction), reduced ballast stress (30%), and extended fatigue life (up to 18.2 times), leading to lower maintenance costs [1,6,11,20,21,57,91].

Alternative blends like DRA provide better storage stability and workability, lowering pavement construction temperatures by 30 °C [34,96,97,98,99]. Rubber–FAM and epoxy–FAM mixtures also enhance crack resistance and energy absorption under fluctuating temperatures [40,41].

Innovations in CRMA applications include rubber composite track supports, waterproofing layers with an elastic modulus of 200–340 MPa [43], and optimized ballast configurations for stable rail performance in temperatures ranging from −10 °C to 45 °C [43,45]. CRMA also contributes to reducing landfill waste, recycling approximately 1500 tires per kilometer of track [41], and lowering emissions by 50% through reduced hydrocarbon output [34].

Further advancements include rubberized concrete with a compressive strength of 59.2 MPa when mixed with silica fume [46] and stabilized ballast for enhanced track longevity [100]. Field studies confirm CRMA’s durability over five years, with AASHTO T-283 tests validating moisture resistance and structural integrity, reinforcing its position as a sustainable transportation solution [45,46,47,60].

Table 7. Performance enhancements of CRMA and related materials in railway infrastructure.

Aspect	Key Findings and Benefits	Implications for Railway Infrastructure
Mechanical Performance and Durability	Enhanced resistance to cracking, rutting, thermal cracking, and fatigue deformations.	Extended service life and reduced maintenance frequency. Improved structural integrity and durability of trackbeds and sleepers. [1,6,11,20,41,42,46,95].
	Increased adhesion with aggregates and higher binder viscosity.
	Optimized CRMA mixtures (e.g., 2% air void content) yield up to 18.2× higher fatigue life.
	Superior load distribution in CRMB sub-ballast; improved bearing capacity and stress distribution.
Vibration Attenuation and Damping	High damping ratio and stiffness effectively reduce vibrations from high-speed trains.	Improved ride quality and track stability. Lower dynamic stresses lead to reduced track degradation and maintenance needs. [1,6,20,21,39,46,57].
	Laboratory tests report up to 61.8% reduction in peak vibration amplitude and 55% lower sleeper vibration velocity.
	Enhanced energy dissipation minimizes dynamic stresses on the track system.
Thermal and Temperature Performance	Increased resistance to temperature fluctuations extends the viscoelastic range of asphalt.	Enhanced performance in extreme climates. Better long-term durability under varying temperature conditions. [34,41,45,47,95].
	Higher softening point and stiffness indicate superior high-temperature performance.
	Lower thermal conductivity and enhanced low-temperature cracking resistance (as seen in DRA) mitigate thermal stresses.
Load Distribution and Structural Integrity	Improved stress distribution reduces vertical stresses in ballast by up to 30%.	More resilient track structure with lower settlement and deformation. Extended lifespan of ballast and subgrade components. [42,43,44,67,91].
	Increased lateral resistance and optimized load distribution reduce ballast degradation and track misalignment.
	TPU/WRP composites offer even load distribution and flexible stress management.
Workability and Additional Benefits	Improved storage stability and lower mixing temperatures (by ~30 °C) enhance workability and reduce energy consumption.	Simplified construction processes and improved long-term performance. Reduced overall maintenance and improved safety of railway infrastructure components. [34,40,45,46,60,100].
	Enhanced energy absorption and crack mitigation in rubberized concrete.
	Additional benefits include improved electrical resistivity, fire safety, and moisture retention.

provides an overview of CRMA’s performance enhancements in railway infrastructure. These improvements include increased mechanical durability, better vibration damping, enhanced thermal resistance, and optimized load distribution. The table highlights CRMA’s role in reducing maintenance frequency, extending infrastructure lifespan, and improving track stability. Workability improvements, such as lower mixing temperatures and enhanced material resilience, also contribute to more sustainable and cost-effective railway construction and maintenance.

3.2.3. Health and Safety Concerns

While CRMA offers numerous engineering benefits, it also presents certain health and performance trade-offs. Studies indicate that CRMA production can emit volatile organic compounds (VOCs) such as p-xylene, toluene, and benzene, particularly at elevated temperatures or in rubber-rich mixtures, potentially affecting workers and nearby communities [1,6,11,20,21,101]. Strategies such as emission inhibitors (e.g., kaolin and expanded graphite) have been explored to mitigate these risks [101].

In terms of performance, CRMA materials exhibit lower aging resistance and elasticity compared to conventional rubberized asphalt (RRA), particularly in desulfurized rubber asphalt (DRA) [34,102,103,104]. Exceeding the optimal rubber content (3–4%) may also reduce stiffness, leading to premature deformation and cracking under dynamic loads [40,41,42,45].

Other materials, such as TPU/WRP composites and WTR-modified ballast, face moisture sensitivity and temperature-dependent elasticity, highlighting the need for optimized mix designs to enhance durability and long-term reliability [43,44]. These findings underscore the importance of comprehensive risk assessments, lifetime testing, and material optimizations to ensure that CRMA remains a safe and effective solution for railway infrastructure.

Table 8. Health and safety concerns of CRMA and related materials in railway applications.

Concern	Key Findings and Risks	Mitigation Strategies/Recommendations
VOC Emissions	Higher rubber content in CRMA can lead to increased emissions of harmful VOCs (e.g., p-xylene, toluene, benzene), especially at elevated temperatures, posing health risks to workers and nearby communities.	Incorporate inhibitors (e.g., kaolin, expanded graphite) to control VOC, NO, and H2S release; conduct comprehensive risk assessments [1,6,11,20,21,101].
Excessive Rubber Content	Excessive rubber (>3–4% in asphalt mixtures; >35% in TPU/WRP) may reduce material stiffness, potentially leading to premature deformation and compromised track stability, which can affect overall safety.	Optimize rubber content to balance flexibility and structural integrity; adjust mix design to ensure adequate stiffness and durability [40,41,42,43,45].
Rubber Migration and Moisture Sensitivity	Loose rubber particles in ballast layers can migrate, interfering with track stability. Additionally, variable permeability of waste tire rubber (WTR) may lead to moisture accumulation and instability under extreme conditions.	Optimize mix designs to prevent rubber migration; evaluate permeability and modify formulations to ensure consistent performance under varying temperatures [44].
Aging and Long-Term Durability	Aging of CRMA mixtures may reduce fatigue life and affect long-term performance, potentially compromising safety over extended service periods.	Further research into modification strategies is needed to enhance aging resistance and maintain mechanical performance over time [11,34].

summarizes key health and safety concerns related to CRMA in railway applications. Issues such as VOC emissions, excessive rubber content, rubber migration, and long-term durability challenges are highlighted alongside potential mitigation strategies. Proper mix design optimization, emissions control, and risk assessments are crucial to ensuring the safe and sustainable use of CRMA in railway infrastructure.

3.3. Case Studies

CRMA has been successfully implemented in various railway projects worldwide, demonstrating improvements in load distribution (United States) [42], reduced wear (Germany) [60], and increased track stability (Italy, Japan) [42,60]. It has also shown significant advantages in durability (Australia) [44], noise reduction (Japan) [33], and vibration damping (Norway) [1,20,49].

Further studies highlight DARC’s effectiveness in vibration damping [39] and the environmental benefits of tire recycling (~300 tires/km) [35]. However, specific challenges exist, such as those noted in Norway’s coastal railway study, which warns of potential maritime-related durability concerns [49]. Despite these challenges, rubberized asphalt has maintained rigidity [28], and recycled railway substructures have contributed to significant energy savings [27].

Projects in Germany, France, and Spain further validate the performance and sustainability benefits of CRMA, reinforcing its role in modern railway infrastructure development [44,60,100].

Table 9. Case studies of CRMA and asphalt-based applications in railway infrastructure.

Case Study/Region	Application/Technique	Key Findings and Benefits
Bridge Approaches and Tunnel Floors	Asphalt underlayment in high-stress areas.	Improved track performance and reduced maintenance costs through enhanced vibration damping [6].
High-Speed Rail in Europe	Use of asphalt underlayment (AUL) and cement asphalt mortar (CAM).	Significant improvements in load distribution, subgrade protection, and noise reduction [16].
Global Integration of Recycled Materials	Incorporation of recycled aggregates and CRMA in substructures.	Achieves energy savings, maintains structural integrity under diverse conditions, and offers competitive performance compared to traditional methods [27].
Laboratory vs. Field Comparative Analysis	Rubberized asphalt testing under simulated railway loads.	Modified mixes demonstrate superior stiffness and durability; laboratory results align with field data [28].
High-Speed Railway Systems	Asphalt concrete support layers.	Reduced track degradation, improved load distribution, and lower dynamic stresses leading to enhanced ride quality and prolonged service life [32].
Porous Asphalt for Noise Reduction	CR-modified porous asphalt mixtures.	Enhanced sound absorption and noise reduction, though mechanical properties must be balanced to ensure long-term durability [33].
Ballasted Tracks—Two-Step Stoneblowing Process	Addition of recycled rubber over stone layers.	Optimizes elastic distribution; reduces rail deflection fluctuations and ballast stress; offers significant environmental and economic benefits (e.g., recycling ~300 tire tread layers/km) [35].
Laboratory Simulation of Sub-Ballast	DARC-based sub-ballast under dynamic load simulation.	Provides better damping and reduced stress concentrations compared to conventional HMA-RFI (hot mix asphalt based on Italian standard), with dynamic deflections maintained due to viscous properties [39].
International Applications—CRMB	Use of CRMB sub-ballast in high-speed rail projects.	Demonstrates long-term stability, effective waterproofing, and reduced track degradation in projects from Italy, Japan, Germany, and the United States [42].
International Applications—WTR Usage	Application of waste tire rubber in rail dampers and under-ballast mats.	Enhances noise and vibration control, improves waterproofing, and reduces maintenance cycles, promoting overall sustainability [44].
Global Asphalt Trackbed Implementations	Asphalt trackbeds for rehabilitation and new construction.	Offers improved track stability, effective load distribution and drainage, and reduced maintenance costs; widely applied in the United States, Japan, Italy, Germany, France, Spain, and Austria [11,60].
Norway—Environ- mental Compatibility	Use of CRG in artificial turf and CRMA in railway applications.	CRG shows acceptable environmental compatibility with contaminants below regulatory limits; CRMA effectively reduces ground vibrations, making it viable for harsh coastal climates in Norway [1,20].

summarizes key case studies showcasing CRMA and asphalt-based applications in railway infrastructure. These projects highlight CRMA’s benefits in track stability, durability, noise reduction, and sustainability, confirming its potential for broader implementation in modern railway systems.

3.4. Global Perspectives on CRMA

Global studies from the United States, Denmark, and China indicate a growing trend toward standardizing CRMA mixing and compaction temperatures, although climate variations significantly affect workability [47]. Practical implementations confirm that CRMA enhances track performance and longevity under diverse conditions [91]. Laboratory research highlights its compatibility with virgin aggregates, while field tests demonstrate improved track resilience and reduced lateral deformations [63,64].

Further innovations include rubber composite sleepers, which reduce ground vibrations by 76%, enhance elasticity, and improve durability, making them a viable alternative to conventional sleepers [57]. Similar trends observed in road construction applications reinforce CRMA’s potential in sustainable transport infrastructure worldwide [57,67,91].

Table 10. Global perspectives on CRMA applications in railway infrastructure.

Aspect	Key Findings and Benefits	Implications for Global Applications
Standardization and Climatic Influence	Global survey reveals challenges in establishing unified mixing and compaction standards for CRMA, with workability significantly affected by regional climatic conditions.	Highlights the need for standardized application methods to optimize performance and ensure safety across diverse climatic regions [47].
Cross-Industry Real-World Applications	Successful integration of waste rubber in railway track structures demonstrates improved performance and longevity.	Provides evidence of CRMA’s feasibility and potential for knowledge transfer to road construction, supporting the adoption of sustainable materials across transportation sectors [91].
Field and Laboratory Validation	Controlled integration of tire-derived aggregate (TDA) under railway sleepers shows enhanced track resilience and reduced lateral deformations under simulated traffic conditions.	Confirms the adaptability of recycled rubber in both railway and road infrastructure, validating its effectiveness under varied climates and traffic conditions worldwide [67].
Rubber Composite Sleepers Evaluation	Full-scale laboratory tests of rubber composite sleepers indicate significant reductions in ground vibrations (up to 76%), improved track elasticity, and long-term durability compared to conventional options.	Demonstrates the global applicability of rubber-modified infrastructure materials and supports the cross-industry adoption of recycled rubber, reinforcing its benefits in enhancing both railway and highway engineering [57].

provides an overview of CRMA applications across different countries, highlighting the benefits and challenges associated with its adoption in railway infrastructure. The table summarizes key advancements in standardization, performance optimization, and sustainability considerations, reflecting CRMA’s potential as a globally viable railway material.

3.5. Laboratory Findings and Implications

Laboratory studies confirm that CRMA significantly enhances railway pavement fatigue life, increasing it by up to 18.2 times in low-stress conditions [11,105]. CRMA exhibits high stiffness and damping properties at low temperatures, making it particularly suitable for high-speed rail applications [1,20].

Centrifuge tests demonstrate that incorporating EPS barriers reduces vibrations by 30% [21]. While DRA enhances mixing viscosity, it exhibits slightly lower aging resistance than RRA [34]. CT scans reveal that epoxy–FAM mixtures provide more stable crack propagation than rubber–FAM [40].

For sub-ballast applications, CRMB reduces settlement by half in poor drainage sites, withstanding up to 100 million load cycles, allowing for a cost-effective 9–14 cm thickness [42]. TPU/WRP layers also improve water resistance and vibration absorption, while WTR ballast dissipates 50% more energy but remains temperature-sensitive [43,44].

Further findings indicate that CRM increases asphalt stiffness and heat resistance, thereby prolonging sub-ballast lifespan [45]. Meanwhile, the porosity of rubber-enhanced sleepers can be optimized with a 10% silica fume–5% rubber mix, balancing damping performance and structural strength [46].

Table 11. Laboratory findings and implications of CRMA for railway infrastructure.

Aspect	Key Findings and Benefits	Implications for Railway Infrastructure
Fatigue Life and Durability	CRMA exhibits significantly higher fatigue life compared to conventional asphalt.	Longer pavement lifespan and reduced maintenance requirements for railway tracks [11,105].
	Reducing air void content from 4% to 2% can increase fatigue life by up to 18.2×.
Vibration Attenuation and Damping	CRMA and RMA show higher stiffness and damping ratios, especially at low temperatures.	Improved track stability and ride quality; effective attenuation of vibrations from high-speed trains [1,20,21].
	Centrifuge tests indicate that CRMA with 1/5 scale aggregates can reduce ground vibrations by up to 30%.
	Enhanced damping improves energy absorption.
Rheological and Thermal Performance	DRA exhibits superior viscosity-temperature behavior, allowing easier mixing and compaction.	Enhanced mix workability, reduced risk of thermal cracking, and consistent performance under fluctuating climatic conditions [34,41,45].
	Optimal rubber content (≈2% by weight) balances durability and workability.
	Better insulation and reduced thermal-induced cracking observed.
	CRM increases stiffness and improves heat resistance, maintaining stability across temperature variations.
Crack Propagation and Fracture Behavior	Acoustic Emission (AE) analysis shows that epoxy–FAM experiences more stable crack propagation than rubber–FAM.	Better understanding of fracture mechanisms leading to optimized mix designs for improved crack resistance [40].
	CT scanning confirms crack initiation at pre-cut slits with gradual microcrack expansion.
	Clustering analysis differentiates between tensile and shear cracking modes.
Track Settlement and Structural Stability	CRMB sub-ballast reduces seasonal vertical displacements by up to 50%.	Reduced settlement and enhanced structural performance of track systems, leading to longer service life and lower maintenance costs [42].
	FEM simulations indicate that CRMB withstands over 100 million load cycles.
	Thickness optimization shows that 9–14 cm of CRMB can replace 30 cm of conventional granular sub-ballast, yielding material and cost savings.
Composite Material Performance	TPU/WRP composites demonstrate reduced water permeability, effective absorption of train-induced vibrations, and structural stability under repeated loading.	Improved waterproofing and stress distribution, resulting in enhanced long-term track stability and reduced subgrade deformation [44].
	WTR-modified ballast reduces track settlement by up to 50% and improves damping, with stiffness varying with temperature.
Additional Enhancements and Mix Optimization	Incorporation of crumb rubber reduces the density of sleepers, making them lighter and easier to transport.	Lighter, more durable sleepers with enhanced impact resistance and energy dissipation, contributing to overall railway infrastructure efficiency and safety [46].
	An optimal mix (10% silica fume + 5% CR) maintains sufficient strength while maximizing damping efficiency.

summarizes key laboratory findings on CRMA’s impact on railway infrastructure. The table highlights improvements in fatigue life, vibration reduction, material stability, and durability, demonstrating CRMA’s suitability for long-term railway application.

3.6. Comparison with Other Materials

CRMA offers significant advantages over traditional railway materials in deformation resistance, flexibility, and fatigue lifespan, with fatigue performance improving by up to 7.2 times compared to conventional mixtures [11]. It also exhibits superior vibration damping, outperforming ballast, concrete, and standard asphalt, with rubber sleepers reducing vibrations by 76% [1,20,21,46,57].

In terms of specific materials:

• DRA (desulfurized rubber asphalt) provides better low-temperature flexibility but has lower rutting resistance compared to CRMA [34].
• PMA (polymer-modified asphalt) exhibits high rutting resistance but is significantly more expensive than CRMA [34].
• RMA (rubber-modified asphalt) enhances fatigue resistance and drainage properties, making it a viable alternative in railway infrastructure [41,42].
• CRMB (crumb rubber-modified bitumen) offers improved waterproofing and better noise/vibration control, making it suitable for urban rail applications [41,42].
• TPU/WRP composites and WTR-modified ballast provide cost-effective vibration damping, particularly in high-speed and heavy-load railway environments [43,44].
• In hot climates, CR-modified asphalt has effectively reduced rutting while maintaining lower costs than PMA, making it a more sustainable option [45].

Although CRMA presents significant sustainability advantages, its potential environmental risks must be carefully addressed. Studies on hexa-methoxy-methyl-melamine [106] and artificial turf materials [107] emphasize the importance of comprehensive risk assessments. While CRMA aids in waste tire recycling, concerns regarding the leaching of toxic compounds require further investigation to ensure safe and sustainable railway applications [49].

Overall, CRMA integrates durability, cost-efficiency, and sustainability, positioning itself as a practical and long-term railway material capable of meeting the demands of modern infrastructure projects.

4. Discussion

The findings of this study highlight the significant advantages of CRMA in railway pavements, particularly in enhancing durability, reducing noise and vibration, and minimizing environmental impact. CRMA has proven to be effective in high-stress areas such as bridge approaches and tunnels, where traditional materials often degrade rapidly under repetitive loading [1]. However, several key areas require further investigation to optimize its application and long-term performance.

One essential research focus is refining non-destructive testing techniques and advanced simulation models to evaluate CRMA’s structural integrity and mechanical performance in modified asphalt-based materials (ABM) and CR trackbeds [6]. Additionally, RMA underlayments require further field testing to validate laboratory findings regarding durability, noise reduction, and fatigue resistance [20,21]. Long-term monitoring programs should be implemented to assess the performance of rubber sleepers and recycled material integration in railway trackbeds [57,67].

Another critical aspect involves optimizing CRMA mix designs, layer thickness, and warm mix additives to improve workability and performance longevity [32]. Further studies should explore the potential of porous asphalt in railway applications, particularly in terms of noise reduction and durability improvements [33]. Additionally, desulfurized rubber asphalt (DRA) should be evaluated for its rutting resistance, life cycle assessments (LCAs), and compatibility with railway substructures [34].

Given the increasing demand for sustainable track maintenance, research must focus on validating the design of DARC (dry asphalt rubber concrete) mixes, ensuring they meet fatigue and load-bearing requirements under railway operational conditions [35,39]. Field trials are necessary to determine optimal rubber content and the role of polymer/nano-additives in rubber-modified asphalt, improving mechanical stability and economic feasibility [40]. Similarly, rubber–asphalt ratios in RMA and CRMB sub-ballast applications require further investigation, particularly in cost–benefit analyses and maintenance reduction strategies [41,42].

Future research should also explore the potential of TPU/WRP blends in hybrid reinforcement systems, which could enhance track flexibility and durability [43]. The use of waste tire rubber (WTR) for ballast performance optimization should be further assessed, particularly regarding its long-term behavior under varying temperature conditions [44]. Additionally, CRM formulations should be examined for extended monitoring, fatigue life improvements, and comprehensive LCAs to determine their long-term environmental and economic benefits [45].

Finally, rubberized sleepers for high-speed railways require extensive research to optimize material composition, economic feasibility, and mechanical resilience in response to dynamic and cyclic loading conditions [46]. The review process itself has limitations that should be acknowledged. The search was restricted to English-language studies, potentially missing relevant research published in other languages. Additionally, the lack of access to 49 reports during the retrieval phase may have excluded valuable studies, as highlighted in the study selection process. The absence of a formal quality assessment, typical for scoping reviews, also means that the methodological rigor of the included studies was not systematically evaluated, which could affect the reliability of some findings.

Table 12. Future research directions for CRMA and related materials in railway infrastructure.

Research Area	Key Topics/Focus	Implications/Goals
Long-Term Field Performance Monitoring	Evaluate the durability, aging, and mechanical degradation of CRMA under real-world traffic and climatic conditions.	Validate laboratory findings, optimize maintenance, and ensure long-term performance [1,11,32,35,39,40,41,42,43,44,45,46,47,57,67,91].
Optimization of Material Composition and Mix Design	Optimize composition, dimensions, and rubber content in CRMA, RMA, DRA, CRMB, TPU/WRP, and rubberized sleepers.	Enhance durability, workability, and cost efficiency while balancing flexibility and stiffness [11,20,21,40,41,42,43,45,46].
Noise Reduction and Vibration Attenuation	Assess and improve noise reduction capabilities and vibration attenuation through field tests and laboratory evaluations.	Achieve better ride quality and reduced environmental noise; further mitigate vibrations in high-speed rail systems [1,21,47,57].
Hybrid Material Innovations and Integration	Explore hybrid modifications by integrating recycled rubber with polymers, nano-additives, fibers, or other sustainable additives.	Develop next-generation, high-performance materials with superior mechanical and environmental properties [27,28,34,40,41,42,43,44,45,46,67,91].
Advanced Testing and Simulation Techniques	Develop non-destructive testing methods and advanced simulation models (FEM, DEM) to predict long-term behavior and optimize material formulations.	Enhance prediction accuracy and streamline material development processes [16,32,44,67,91].
Economic Feasibility and Life Cycle Assessments	Conduct comprehensive life cycle assessment (LCA) and life cycle cost analysis (LCCA) studies to quantify environmental and economic benefits.	Provide quantifiable evidence for sustainable adoption and support policy decisions in railway infrastructure [27,33,34,41,42,43,44,45,46,57,67,91].
Adhesion and Environmental Impact Studies	Investigate innovative methods to enhance the adhesion properties of crumb rubber in asphalt mixtures; assess long-term environmental and health impacts.	Improve pavement performance while mitigating potential health and environmental risks [21,47].

outlines future research directions, emphasizing key areas that require further investigation to enhance the performance, sustainability, and cost-effectiveness of CRMA in railway infrastructure.

5. Conclusions

Using crumb rubber-modified asphalt (CRMA) in railway infrastructure marks a significant step toward more sustainable and durable trackbed systems. This review highlights how CRMA improves track performance by enhancing vibration damping, optimizing load distribution, and increasing resistance to fatigue and thermal cracking. Beyond its engineering benefits, CRMA also contributes to environmental sustainability by repurposing waste tires, reducing pollution, and lowering dependency on virgin materials—aligning with global circular economy objectives. Case studies from various railway applications, including high-speed networks, freight corridors, and urban transit systems, demonstrate its adaptability and long-term viability under different climatic and operational conditions.

However, several challenges remain, such as VOC emissions during production, variations in rubber–asphalt compatibility, and uncertainties regarding long-term performance in extreme weather conditions. Future research should explore hybrid composites (e.g., TPU/WRP blends) and advanced desulfurization techniques to enhance CRMA’s mechanical and chemical properties. Additionally, optimizing rubber content, refining mix designs, and employing predictive modeling methods like FEM and DEM will be crucial for accurately assessing performance and enabling large-scale implementation.

Widespread adoption of CRMA in railway infrastructure will require strong collaboration between researchers, industry stakeholders, and policymakers. Large-scale field trials, standardized engineering guidelines, and thorough life cycle assessments will be essential to validating its economic and environmental benefits. By bridging the gap between laboratory research and real-world applications, CRMA has the potential to transform railway infrastructure, offering a resilient, low-carbon solution that meets the demands of modern transportation while supporting global sustainability goals. As the railway sector moves toward greener and more advanced infrastructure, CRMA stands out as a key material that balances engineering performance, environmental responsibility, and long-term cost efficiency.

Future research will aim to address the following:

• Optimization of hybrid material compositions [108,109,110,111]: investigate the integration of recycled rubber with polymers, nano-additives, and sustainable reinforcements to enhance durability, workability, and mechanical resilience;
• Implementation of non-destructive testing (NDT) and AI-driven simulations and optimizations: develop advanced non-destructive testing (NDT) techniques and AI-powered predictive models (e.g., FEM, DEM) [112,113] to evaluate material performance, optimize mix designs, and assess long-term structural integrity;
• Improve efficiency with special optimizations, like fuzzy and similar solutions [114,115,116,117,118];
• Environmental and life cycle assessments: conduct comprehensive LCA and life cycle cost analysis (LCCA) [119,120,121,122,123] to quantify the economic and environmental benefits of CRMA applications, particularly in railway infrastructure;
• Enhancement of noise and vibration mitigation strategies: investigate the effectiveness of porous rubberized asphalt and TPU/WRP composites for improved vibration damping [124,125,126] and noise reduction [127,128] in high-speed and heavy-load railway environments [12,13].