State-of-the-Art Review on Additive Manufacturing Technology in Railway Infrastructure Systems

: Additive manufacturing technologies, well known as three-dimensional printing (3DP) technologies, have been applied in many industrial ﬁelds, including aerospace, automobiles, ship-building, civil engineering and nuclear power. However, despite the high material utilization and the ability to rapidly construct complex shaped structures of 3D printing technologies, the application of additive manufacturing technologies in railway track infrastructure is still at the exploratory stage. This paper reviews the state-of-the-art research of additive manufacturing technologies related the railway track infrastructure and discusses the challenges and prospects of 3D printing technology in this area. The insights will not only help the development of 3D printing technologies into railway engineering but also enable smarter railway track component design and improve track performance and inspection strategies.


Introduction
The railway industry is a traditional part of civil engineering.Since the first railway line was built in 1825, the railway system has been one of the most important transportation methods in every country around the world [1].The railway includes tram, metro, intercity, freight railway and passenger railways (normal-speed and high-speed).According to the type of supporting layer, railway systems can be classified into ballasted track and ballast-less track (slab track) [2].The ballasted railway track is usually composed of the rails (including stock rails, closure rails, wing rails and guardrails in turnout area), fastener system, sleepers (bears in crossing area), ballasted bed (including ballast layer and subballast layer) and subgrade [3].In slab track railway systems, the concrete track slab serves as a substitution for the sleepers and the ballasted layer to support the rails [4,5].Ballasted tracks are more widely used than concrete slab tracks because of their easy maintenance and low construction cost.Concrete slab tracks are adopted in high-speed railway lines for high track stiffness and good ride comfort [5].The railway track system also includes special components for particular issues, for example, retaining walls in bridge railways [6], geogrids for locking ballast particles [7], soundproof walls for noise reduction [8] and sand-blocking fences for preventing the sand from damaging the track components [9].The railway tracks (including ballasted track and slab track) are an assembly of many structures, working collaboratively to keep the rail gauge and rail elevation.
The components of ballasted track and concrete slab track systems are present in Figure 1.Railway infrastructure can redistribute the dynamic loads induced by passing trains, prevent rails from deforming and moving, reduce noise and improve ride comfort [2].The stress inside railway infrastructure is greatest near the wheel-rail interface and is redistributed to a lower value on the rail-to-sleeper (including embedded sleeper in slab track) contact surface with a larger contact area.The dynamic loads are also redistributed at the sleeper to ballasted layer, ballasted layer to subgrade, embedded sleeper to concrete slab and concrete slab to subgrade interfaces.In order to serve in safe and comfortable conditions, railway infrastructure is designed with strong parts (e.g., rails and sleepers) on the top and weak parts (e.g., ballasted layer with crushed rocks and subgrade) on the sublayer.This can be observed from the strength of materials used for the components.For example, the mechanical strengths of rail (steel), sleeper (concrete, timber, composite and steel), ballasted layer (crushed rock particles) and subgrade (soil) are from high to low, accordingly.
J. Compos.Sci.2022, 6, x FOR PEER REVIEW 2 of 22 in slab track) contact surface with a larger contact area.The dynamic loads are also redistributed at the sleeper to ballasted layer, ballasted layer to subgrade, embedded sleeper to concrete slab and concrete slab to subgrade interfaces.In order to serve in safe and comfortable conditions, railway infrastructure is designed with strong parts (e.g., rails and sleepers) on the top and weak parts (e.g., ballasted layer with crushed rocks and subgrade) on the sublayer.This can be observed from the strength of materials used for the components.For example, the mechanical strengths of rail (steel), sleeper (concrete, timber, composite and steel), ballasted layer (crushed rock particles) and subgrade (soil) are from high to low, accordingly.
In recent decades, ultra-high-speed and heavier-haul railways have been developed in many countries (such as China, France and Japan) [10].The rolling stocks with higher speed and larger mass can induce dynamic axle load with greater magnitude and higher frequency than in the past, which puts forward stricter requirements on the reliability of the railway systems than before.Most recently, the railway turnout system's safety and reliability are becoming a major problem due to the aforementioned trends.The failure and deterioration of the railway infrastructures caused by the train passing, the environmental variation, and the deterioration of ballasted layers is more frequent than before.The typical components of the railway turnout system, such as rail pads, concrete bearers and ballasted beds, were designed to support the rolling stocks with normal speed and mass.Those components may fail when subjected to the current complex load, which results in a great risk to comfort and safety.In extreme cases, it may lead to safety incidents such as derailment and roll-over.The performance of conventional railway components is limited.Also, the severe loads can result in wheel and rail wear and difficulty in track detection.Thus, it is of great significance to design components with better energy absorption and vibration reduction performance given the current railway serving load.Many researchers have phased in new technologies to improve conventional railway track systems, such as machine learning [11], additive manufacturing [12], 3D scanning [13,14], topology optimization [15], etc.
Additive manufacturing technology, well known as 3D printing, is based on digital model files using layer-by-layer addition of metal powders or plastic materials to fabricate three-dimensional objects [16,17].According to ASTM Standard F2792 [18], the 3D printing technologies can be classified by the processing similarities into seven groups: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination and vat photopolymerization.Their main principles are layered manufacturing and layered overlaying.As a product of the fusion of machinery, materials, computer science and other technologies, additive manufacturing technology has made direct manufacturing driven by digital models a reality and has become a landmark technology for the new industrial revolution [19,20].Compared with traditional manufacturing technology, 3D printing enables the integration of complex structures and mould-free manufacturing, personalized customization, multi-components and multi-materials, while reducing material waste [21].As reported in [22], 3D printing technologies have been widely used in many fields including industrial machines, consumer products/electronics, In recent decades, ultra-high-speed and heavier-haul railways have been developed in many countries (such as China, France and Japan) [10].The rolling stocks with higher speed and larger mass can induce dynamic axle load with greater magnitude and higher frequency than in the past, which puts forward stricter requirements on the reliability of the railway systems than before.Most recently, the railway turnout system's safety and reliability are becoming a major problem due to the aforementioned trends.The failure and deterioration of the railway infrastructures caused by the train passing, the environmental variation, and the deterioration of ballasted layers is more frequent than before.The typical components of the railway turnout system, such as rail pads, concrete bearers and ballasted beds, were designed to support the rolling stocks with normal speed and mass.Those components may fail when subjected to the current complex load, which results in a great risk to comfort and safety.In extreme cases, it may lead to safety incidents such as derailment and roll-over.The performance of conventional railway components is limited.Also, the severe loads can result in wheel and rail wear and difficulty in track detection.Thus, it is of great significance to design components with better energy absorption and vibration reduction performance given the current railway serving load.Many researchers have phased in new technologies to improve conventional railway track systems, such as machine learning [11], additive manufacturing [12], 3D scanning [13,14], topology optimization [15], etc.
Additive manufacturing technology, well known as 3D printing, is based on digital model files using layer-by-layer addition of metal powders or plastic materials to fabricate three-dimensional objects [16,17].According to ASTM Standard F2792 [18], the 3D printing technologies can be classified by the processing similarities into seven groups: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination and vat photopolymerization.Their main principles are layered manufacturing and layered overlaying.As a product of the fusion of machinery, materials, computer science and other technologies, additive manufacturing technology has made direct manufacturing driven by digital models a reality and has become a landmark technology for the new industrial revolution [19,20].Compared with traditional manufacturing technology, 3D printing enables the integration of complex structures and mould-free manufacturing, personalized customization, multi-components and multi-materials, while reducing material waste [21].As reported in [22], 3D printing technologies have been widely used in many fields including industrial machines, consumer products/electronics, aerospace, medical/dental, academic, government/military, architectural and etc. Figure 2 presents the proportions of 3D printing technologies adopted in different fields.The government and academic institutions have shown growing interest in 3D printing technologies for the ability to produce complex objects and the increasing 3D printing market all over the world [23].In the last few decades, many railway institutions and researchers started to combine railway industry with 3D printing technologies.For example, Germany Siemens Mobility has established the Alliance for Availability system and plan to 3D print spare parts for railway vehicles; Mobility Goes Additive (MGA) has printed the first safety component for the railway sectors; the China Academy of Railway Sciences (CARS) established the 3D printing lab in 2016 for phasing 3D printing into track structures.aerospace, medical/dental, academic, government/military, architectural and etc. Figure 2 presents the proportions of 3D printing technologies adopted in different fields.The government and academic institutions have shown growing interest in 3D printing technologies for the ability to produce complex objects and the increasing 3D printing market all over the world [23].In the last few decades, many railway institutions and researchers started to combine railway industry with 3D printing technologies.For example, Germany Siemens Mobility has established the Alliance for Availability system and plan to 3D print spare parts for railway vehicles; Mobility Goes Additive (MGA) has printed the first safety component for the railway sectors; the China Academy of Railway Sciences (CARS) established the 3D printing lab in 2016 for phasing 3D printing into track structures.This paper focuses on additive manufacturing technology (3D printing) application in railway track infrastructure components.The state-of-art review has summarised the 3D printing technology about railway track research in the following contents, by presenting the relevant research on the rails, fasteners, rail pads, sleepers(bearers), supporting layers (ballasted bed and concrete track slab) and special components.The challenges of adopting additive manufacturing to produce railway infrastructures are discussed.The future prospects of 3D printing technologies in railway infrastructure component design and improving track detection and maintenance are concluded.The insight can promote the development of additive manufacturing technology in railway engineering.Furthermore, this will help to put forward new solutions for addressing some railway problems, such as railway track stiffness issues in transition zones and rail/wheel wear problems.This paper focuses on additive manufacturing technology (3D printing) application in railway track infrastructure components.The state-of-art review has summarised the 3D printing technology about railway track research in the following contents, by presenting the relevant research on the rails, fasteners, rail pads, sleepers(bearers), supporting layers (ballasted bed and concrete track slab) and special components.The challenges of adopting additive manufacturing to produce railway infrastructures are discussed.The future prospects of 3D printing technologies in railway infrastructure component design and improving track detection and maintenance are concluded.The insight can promote the development of additive manufacturing technology in railway engineering.Furthermore, this will help to put forward new solutions for addressing some railway problems, such as railway track stiffness issues in transition zones and rail/wheel wear problems.

Additive Manufacturing in Railway Components
The railway track infrastructure is a complex system composed of multiple components.Each component must have sufficient strength, elasticity and durability to provide adequate lateral and longitudinal resistance and proper track stiffness.In this section, the research about each component related to additive manufacturing technology is reviewed.

Rails
The rail directly supports the rolling stock and guides the direction of passing trains.The dynamic loads induced by the moving vehicles can result in rail corrugation, wear, rolling contact fatigue (RCF) issues and surface damage [24][25][26].With the rapid development of high-speed and heavy-haul railway lines, the risk of rail damage and failure is increasing [27,28].In order to improve the rail wear and RCF resistance, the select laser melting(SLM) additive manufacturing technology, also called laser melting, have been adopted by many researchers to produce an enhanced layer on the rail top surfaces [29].
The European InfraStar project first used the laser cladding technology to produce a reinforced layer on the railhead area [30][31][32][33], as shown in Figure 3a.The noise measurements of the enhanced bi-material rails with different Duroc materials were then carried out in Malmbanan (Figure 3b).This approach can increase rolling contact fatigue performance, shorten the maintenance intervals and reduce the noise caused by wheel-rail interaction [30].Compared with past repairing technology, the additive manufacturing not only can repair the local real damage but also can improve the rail performance and prolong rail service life [34].
vide adequate lateral and longitudinal resistance and proper track stiffness.In this section, the research about each component related to additive manufacturing technology is reviewed.

Rails
The rail directly supports the rolling stock and guides the direction of passing trains.The dynamic loads induced by the moving vehicles can result in rail corrugation, wear, rolling contact fatigue (RCF) issues and surface damage [24][25][26].With the rapid development of high-speed and heavy-haul railway lines, the risk of rail damage and failure is increasing [27,28].In order to improve the rail wear and RCF resistance, the select laser melting(SLM) additive manufacturing technology, also called laser melting, have been adopted by many researchers to produce an enhanced layer on the rail top surfaces [29].
The European InfraStar project first used the laser cladding technology to produce a reinforced layer on the railhead area [30][31][32][33], as shown in Figure 3a.The noise measurements of the enhanced bi-material rails with different Duroc materials were then carried out in Malmbanan (Figure 3b).This approach can increase rolling contact fatigue performance, shorten the maintenance intervals and reduce the noise caused by wheelrail interaction [30].Compared with past repairing technology, the additive manufacturing not only can repair the local real damage but also can improve the rail performance and prolong rail service life [34].Figure 4 shows the schematic of the laser cladding process.The laser cladding process includes: (1) a molten pool is built on the substrate (railhead) using laser beam; (2) metallic powders are delivered to the molten pool area by carrier gas; (3) the metallic powders are molten, fused with the base substrate in molten pool and cooled down; (4) the nozzles and laser beam are moved through a certain routine controlled by the computer.The processes are repeated [35,36].
The mechanical properties (e.g., tensile strength and residual stress) of laser cladded rail is affected by the material type, the granularity of alloy powders and 3D printing processes (e.g., scanning pattern, designed angle, printing speed and laser power) [37,38].The laser cladding can be used to enhance many types of rails, including hyper-eutectoid rail, U71Mn rail (GB), R400HT(EN), HE400(EN), R260(EN), R200(EN).The cladded layer can be classified into Fe-based, Ni-based, Co-based and other types according to the base material type.The summary of cladding materials for enhancing rails and the related 3D printing process are listed in Table 1.The influence of laser cladding on the rail has not been fully understood.However, it can be concluded from Figure 4 shows the schematic of the laser cladding process.The laser cladding process includes: (1) a molten pool is built on the substrate (railhead) using laser beam; (2) metallic powders are delivered to the molten pool area by carrier gas; (3) the metallic powders are molten, fused with the base substrate in molten pool and cooled down; (4) the nozzles and laser beam are moved through a certain routine controlled by the computer.The processes are repeated [35,36].the present research that: (1) increasing laser power can help create fine cladding grain [39,40].(2)The size of cladded layer is significantly affected by the powder feeding speed [41,42].(3) The grain size and morphology of the cladding layer are determined by the thermal history [38,43].The high G/R, the ratio of temperature gradient at the solid-liquid interface (G) to thermal gradient cooling rate(R), can promote the occurrence of columnar crystal [24].The low G/R can help the generation of dendrite crystal [44][45][46].(4) Angular microstructures can provide higher hardness than the leaf-like microstructures [47].( 5) Laser cladding can increase the hardness of rail.The hardness increases to the max at around 0.25 mm along the depth direction in the cladded layer and then decreases to the same as the substrate rail [48].( 6) The hardness of cladded rail is not sensitive to small changes in laser cladding processes [49].(7) The higher content of MxCy phases and martensitic structure can make the rail cladded layer harder [50].(8) Increasing the hardness of rail can prevent wear and crack occurrence on the rail-wheel interface [51,52].(9) The thermal processing(heating and cooling) in laser cladding can produce residual stress inside the rail, which will damage rail fatigue and anti-cracking performance [53][54][55].(10) Post-heat treatment to the cladding layer can improve ductility and elongation [56].(11) The bending strength of the cladding layer is better than the substrate rail [28], and the formation of fine pearlite microstructures can benefit the bending behaviour [50,57].(12) The fatigue performance of cladding layers is determined by the properties of the heat affected zone.The laser cladded layer has a uniform deformation when exposed to dynamic load, which can restrain the crack and rail surface failure [48,[58][59][60].(13) Adding reinforcement materials into the cladding layer can help improve the mechanical properties of the rail [61].( 14) Optimization of additive manufacturing processes, such as a magnetic approach, WAAM and HF-WAAM, can help improve the yield strength and ultimate tensile strength of rails [62][63][64][65].[66] Fe-based martensitic stainless steel (MSS) The mechanical properties (e.g., tensile strength and residual stress) of laser cladded rail is affected by the material type, the granularity of alloy powders and 3D printing processes (e.g., scanning pattern, designed angle, printing speed and laser power) [37,38].The laser cladding can be used to enhance many types of rails, including hyper-eutectoid rail, U71Mn rail (GB), R400HT(EN), HE400(EN), R260(EN), R200(EN).The cladded layer can be classified into Fe-based, Ni-based, Co-based and other types according to the base material type.The summary of cladding materials for enhancing rails and the related 3D printing process are listed in Table 1.The influence of laser cladding on the rail has not been fully understood.However, it can be concluded from the present research that: (1) increasing laser power can help create fine cladding grain [39,40].(2)The size of cladded layer is significantly affected by the powder feeding speed [41,42].(3) The grain size and morphology of the cladding layer are determined by the thermal history [38,43].The high G/R, the ratio of temperature gradient at the solid-liquid interface (G) to thermal gradient cooling rate(R), can promote the occurrence of columnar crystal [24].The low G/R can help the generation of dendrite crystal [44][45][46].(4) Angular microstructures can provide higher hardness than the leaf-like microstructures [47].( 5) Laser cladding can increase the hardness of rail.The hardness increases to the max at around 0.25 mm along the depth direction in the cladded layer and then decreases to the same as the substrate rail [48].(6) The hardness of cladded rail is not sensitive to small changes in laser cladding processes [49].(7) The higher content of MxCy phases and martensitic structure can make the rail cladded layer harder [50].(8) Increasing the hardness of rail can prevent wear and crack occurrence on the rail-wheel interface [51,52].(9) The thermal processing(heating and cooling) in laser cladding can produce residual stress inside the rail, which will damage rail fatigue and anti-cracking performance [53][54][55].(10) Post-heat treatment to the cladding layer can improve ductility and elongation [56].(11) The bending strength of the cladding layer is better than the substrate rail [28], and the formation of fine pearlite microstructures can benefit the bending behaviour [50,57].(12) The fatigue performance of cladding layers is determined by the properties of the heat affected zone.The laser cladded layer has a uniform deformation when exposed to dynamic load, which can restrain the crack and rail surface failure [48,[58][59][60].(13) Adding reinforcement materials into the cladding layer can help improve the mechanical properties of the rail [61].( 14) Optimization of additive manufacturing processes, such as a magnetic approach, WAAM and HF-WAAM, can help improve the yield strength and ultimate tensile strength of rails [62][63][64][65].Ref. [73] adopted the fused deposition modelling method to fabricate composite rails for micro-people movers (MPM) and evaluated the failure modes of the printed rails with different polylactide fibre contents (material density) under static load tests.The size of the printed rail and the test settings are present in Figure 5.The results indicate that the bearing capacity of the 3D printed rails is determined by the fibre contents and the printing direction, and the failure modes change from material-control to structure-control as the fibre contents increase from 20% to 100%.[24] Co-based Not mentioned Wang et al. ( 2014) [48] Co-based Not mentioned Aladesanmi et al. (2019) [72] Others Ti, TiB2 Ref. [73] adopted the fused deposition modelling method to fabricate composite rails for micro-people movers (MPM) and evaluated the failure modes of the printed rails with different polylactide fibre contents (material density) under static load tests.The size of the printed rail and the test settings are present in Figure 5.The results indicate that the bearing capacity of the 3D printed rails is determined by the fibre contents and the printing direction, and the failure modes change from material-control to structure-control as the fibre contents increase from 20% to 100%.

Fasteners
The railway fastener system helps to fix rails on sleepers or track slabs and provides elasticity for the rails [74,75].There are many types of fastener system all over the world, such as 102 and 8 K types from Japan, Nabla from French [76], RST and VOSSLOH 300 from German [77,78], Pandrol series from the UK [79] and WJ series from China [80].Their components and structures are different, but they can be regarded as comprising three parts: withholding parts (e.g., clips, clamps), jointing parts (e.g., dog spikes, screw spikes and dowels) and elastic pads (rail pads).The railway fastener systems develop from simple shapes to complex structures, as shown in Figure 6.The fastener systems can be classified into rigid type (Dog spike and KPO type) and elastic type (Shrapnel type

Fasteners
The railway fastener system helps to fix rails on sleepers or track slabs and provides elasticity for the rails [74,75].There are many types of fastener system all over the world, such as 102 and 8 K types from Japan, Nabla from French [76], RST and VOSSLOH 300 from German [77,78], Pandrol series from the UK [79] and WJ series from China [80].Their components and structures are different, but they can be regarded as comprising three parts: withholding parts (e.g., clips, clamps), jointing parts (e.g., dog spikes, screw spikes and dowels) and elastic pads (rail pads).The railway fastener systems develop from simple shapes to complex structures, as shown in Figure 6.The fastener systems can be classified into rigid type (Dog spike and KPO type) and elastic type (Shrapnel type and Spring type).The elastic fasteners provide greater elasticity, better fatigue performance and higher corrosion resistance than rigid type fasteners and have become the most widely used fastener systems in current railway lines.
technology to produce sand-casting moulds for helping fabricate steel fastener plates [81], as present in Figure 7a.Compared with traditional methods, the 3D printing process had shortened the manufacturing time to about 25 days and decreased the cost.Moreover, the 3D printing sand-casting moulds can help the cooling process by the embedded water cooling path, as shown in Figure 7b.The water-cooling path was designed to follow the fastener clip shape, which can make the cooling more effective and reduce the sample deformation.This approach can reduce the moulding time from 150-190 s to 124.5 s.CARS also used the 3D printing technology to optimize the geometry shape of the clip and the crews of the railway fastener system, as shown in Figure 8a,b [81].The design can decrease material usage, increase elasticity by a special shape, and increase mechanical strength and chemical resistance by combining high-performance materials.Also, reference [82] tested the reinforcing effects of 3D printed bolts on rocks.However, it is easy for the fastener systems to fail or be damaged under long-term railway dynamic loads, especially on high-speed and heavy-haul railway lines.In order to design a new type of railway fastener, the CARS in China has adopted 3D printing technology to produce sand-casting moulds for helping fabricate steel fastener plates [81], as present in Figure 7a.Compared with traditional methods, the 3D printing process had shortened the manufacturing time to about 25 days and decreased the cost.Moreover, the 3D printing sand-casting moulds can help the cooling process by the embedded water cooling path, as shown in Figure 7b.The water-cooling path was designed to follow the fastener clip shape, which can make the cooling more effective and reduce the sample deformation.This approach can reduce the moulding time from 150-190 s to 124.5 s.CARS also used the 3D printing technology to optimize the geometry shape of the clip and the crews of the railway fastener system, as shown in Figure 8a,b [81].The design can decrease material usage, increase elasticity by a special shape, and increase mechanical strength and chemical resistance by combining high-performance materials.Also, reference [82] tested the reinforcing effects of 3D printed bolts on rocks.
and Spring type).The elastic fasteners provide greater elasticity, better fatigue performance and higher corrosion resistance than rigid type fasteners and have become the most widely used fastener systems in current railway lines.
However, it is easy for the fastener systems to fail or be damaged under long-term railway dynamic loads, especially on high-speed and heavy-haul railway lines.In order to design a new type of railway fastener, the CARS in China has adopted 3D printing technology to produce sand-casting moulds for helping fabricate steel fastener plates [81], as present in Figure 7a.Compared with traditional methods, the 3D printing process had shortened the manufacturing time to about 25 days and decreased the cost.Moreover, the 3D printing sand-casting moulds can help the cooling process by the embedded water cooling path, as shown in Figure 7b.The water-cooling path was designed to follow the fastener clip shape, which can make the cooling more effective and reduce the sample deformation.This approach can reduce the moulding time from 150-190 s to 124.5 s.CARS also used the 3D printing technology to optimize the geometry shape of the clip and the crews of the railway fastener system, as shown in Figure 8a,b [81].The design can decrease material usage, increase elasticity by a special shape, and increase mechanical strength and chemical resistance by combining high-performance materials.Also, reference [82] tested the reinforcing effects of 3D printed bolts on rocks.Rail pads in fastener systems provide the most elasticity and can reduce the magnitude of the wheel-rail contact force [83,84].The dynamic performance of railway fastener systems is affected by the pad material, thickness and geometric shape [85].Thus, the 3D printing technology can be used to produce elastic pads with enough strength and stiffness for railway fastener systems.There is no report directly related to 3D printed rail pads, but similar research in 3D printed elastic pads has been investigated in aerospace members and civil engineering fields.For example, the cellular honeycomb structures have high specific strength and energy absorption capacity [86].The stiffness of the honeycomb structure can be designed by a given thickness [87][88][89], which can match different railway line requirements.These honeycomb structures have already been adopted in fabricating aeroplanes to replace conventional metallic components for noise reduction, impact load reduction and thermal control [90,91].Lattice structures are also a prospecting structure for rail pads.The lattice structures have good vibration properties [92][93][94], high indentation resistance and greater mechanical strength and stiffness when exposed to bending moments and compressive loads [95].Moreover, by structural design, the lattice structures can gain a negative Poisson's ratio [96].Triply periodic minimal surface (TPMS) structures are newly developed porous structures inspired by natural creatures [97,98].Those TPMS structures have higher specific strength and stiffness compared to common lattice and honeycomb structures [99].Examples of honeycomb structures, lattice structures and TPMS structures are present in Figure 9.These structures have the potential to be used as elastic sandwich layers in rail pads for their designable stiffness and high specific strength.Rail pads in fastener systems provide the most elasticity and can reduce the magnitude of the wheel-rail contact force [83,84].The dynamic performance of railway fastener systems is affected by the pad material, thickness and geometric shape [85].Thus, the 3D printing technology can be used to produce elastic pads with enough strength and stiffness for railway fastener systems.There is no report directly related to 3D printed rail pads, but similar research in 3D printed elastic pads has been investigated in aerospace members and civil engineering fields.For example, the cellular honeycomb structures have high specific strength and energy absorption capacity [86].The stiffness of the honeycomb structure can be designed by a given thickness [87][88][89], which can match different railway line requirements.These honeycomb structures have already been adopted in fabricating aeroplanes to replace conventional metallic components for noise reduction, impact load reduction and thermal control [90,91].Lattice structures are also a prospecting structure for rail pads.The lattice structures have good vibration properties [92][93][94], high indentation resistance and greater mechanical strength and stiffness when exposed to bending moments and compressive loads [95].Moreover, by structural design, the lattice structures can gain a negative Poisson's ratio [96].Triply periodic minimal surface (TPMS) structures are newly developed porous structures inspired by natural creatures [97,98].Those TPMS structures have higher specific strength and stiffness compared to common lattice and honeycomb structures [99].Examples of honeycomb structures, lattice structures and TPMS structures are present in Figure 9.These structures have the potential to be used as elastic sandwich layers in rail pads for their designable stiffness and high specific strength.Rail pads in fastener systems provide the most elasticity and can reduce the magnitude of the wheel-rail contact force [83,84].The dynamic performance of railway fastener systems is affected by the pad material, thickness and geometric shape [85].Thus, the 3D printing technology can be used to produce elastic pads with enough strength and stiffness for railway fastener systems.There is no report directly related to 3D printed rail pads, but similar research in 3D printed elastic pads has been investigated in aerospace members and civil engineering fields.For example, the cellular honeycomb structures have high specific strength and energy absorption capacity [86].The stiffness of the honeycomb structure can be designed by a given thickness [87][88][89], which can match different railway line requirements.These honeycomb structures have already been adopted in fabricating aeroplanes to replace conventional metallic components for noise reduction, impact load reduction and thermal control [90,91].Lattice structures are also a prospecting structure for rail pads.The lattice structures have good vibration properties [92][93][94], high indentation resistance and greater mechanical strength and stiffness when exposed to bending moments and compressive loads [95].Moreover, by structural design, the lattice structures can gain a negative Poisson's ratio [96].Triply periodic minimal surface (TPMS) structures are newly developed porous structures inspired by natural creatures [97,98].Those TPMS structures have higher specific strength and stiffness compared to common lattice and honeycomb structures [99].Examples of honeycomb structures, lattice structures and TPMS structures are present in Figure 9.These structures have the potential to be used as elastic sandwich layers in rail pads for their designable stiffness and high specific strength.

Sleepers
Sleepers support the rail and redistribute the loads to the ballasted layer.The sleepers can be classified into timber, concrete and steel sleepers according to the materials.Concrete sleepers have been widely adopted for their vast resources, good elasticity, long life cycle and good chemical resistance [102].The optimization of present sleepers is conducted from the material aspects.For example, by adding fibres and rubber particles into composite and concrete sleepers, the mechanical performance of sleepers can be improved.Composite sleepers were newly developed in the 1980s, and the base materials include glass fibre, polyurethane, rubber and resin [103,104].Because the materials are recyclable and have good elasticity and insulation, the market share of composite sleepers is increasing year by year.The shift2rail project has tried using 3D printing technology to produce a composite bearer with multi-layers [105], as shown in Figure 10.Layer 1 should exceed 150 mm to provide enough stiffness and space for installing fastener systems.layer 3 (soft layer) works as an under-sleeper pad; its depth should exceed 10 mm for 63 mm ballast, 8 mm for 53 mm ballast and 6 mm for ballast less than 53 mm.The depth of layer 2 is determined by the balance of the total depth of bears, layer 1 and layer 3. The required Young's modulus and density for Layer 1, 2 and 3 decreases as the depth increases.
rials.Concrete sleepers have been widely adopted for their vast resources, good elasticity, long life cycle and good chemical resistance [102].The optimization of present sleepers is conducted from the material aspects.For example, by adding fibres and rubber particles into composite and concrete sleepers, the mechanical performance of sleepers can be improved.Composite sleepers were newly developed in the 1980s, and the base materials include glass fibre, polyurethane, rubber and resin [103,104].Because the materials are recyclable and have good elasticity and insulation, the market share of composite sleepers is increasing year by year.The shift2rail project has tried using 3D printing technology to produce a composite bearer with multi-layers [105], as shown in Fig- ure 10.Layer 1 should exceed 150 mm to provide enough stiffness and space for installing fastener systems.layer 3 (soft layer) works as an under-sleeper pad; its depth should exceed 10 mm for 63 mm ballast, 8 mm for 53 mm ballast and 6 mm for ballast less than 53 mm.The depth of layer 2 is determined by the balance of the total depth of bears, layer 1 and layer 3. The required Young's modulus and density for Layer 1, 2 and 3 decreases as the depth increases.
For timber sleepers and concrete sleepers, no research related to additive manufacturing technologies has been conducted.Some researchers have conducted shape optimisation of the sleepers, such as sleepers with arrowhead grooves [106], sleepers with bottom textures [107] and winged sleepers [108,109], as shown in Figure 11.The sleeper can distribute loads more evenly by optimising the geometric shape and providing better lateral and longitudinal resistance [107,108].It should be noted that producing sleepers with complex shapes using a traditional moulding method is difficult; prototype sleepers with complex shapes for research purposes can be fabricated easily using 3D printing process.For timber sleepers and concrete sleepers, no research related to additive manufacturing technologies has been conducted.Some researchers have conducted shape optimisation of the sleepers, such as sleepers with arrowhead grooves [106], sleepers with bottom textures [107] and winged sleepers [108,109], as shown in Figure 11.The sleeper can distribute loads more evenly by optimising the geometric shape and providing better lateral and longitudinal resistance [107,108].It should be noted that producing sleepers with complex shapes using a traditional moulding method is difficult; prototype sleepers with complex shapes for research purposes can be fabricated easily using 3D printing process.

Sleepers
Sleepers support the rail and redistribute the loads to the ballasted layer.The sleepers can be classified into timber, concrete and steel sleepers according to the materials.Concrete sleepers have been widely adopted for their vast resources, good elasticity, long life cycle and good chemical resistance [102].The optimization of present sleepers is conducted from the material aspects.For example, by adding fibres and rubber particles into composite and concrete sleepers, the mechanical performance of sleepers can be improved.Composite sleepers were newly developed in the 1980s, and the base materials include glass fibre, polyurethane, rubber and resin [103,104].Because the materials are recyclable and have good elasticity and insulation, the market share of composite sleepers is increasing year by year.The shift2rail project has tried using 3D printing technology to produce a composite bearer with multi-layers [105], as shown in Fig- ure 10.Layer 1 should exceed 150 mm to provide enough stiffness and space for installing fastener systems.layer 3 (soft layer) works as an under-sleeper pad; its depth should exceed 10 mm for 63 mm ballast, 8 mm for 53 mm ballast and 6 mm for ballast less than 53 mm.The depth of layer 2 is determined by the balance of the total depth of bears, layer 1 and layer 3. The required Young's modulus and density for Layer 1, 2 and 3 decreases as the depth increases.
For timber sleepers and concrete sleepers, no research related to additive manufacturing technologies has been conducted.Some researchers have conducted shape optimisation of the sleepers, such as sleepers with arrowhead grooves [106], sleepers with bottom textures [107] and winged sleepers [108,109], as shown in Figure 11.The sleeper can distribute loads more evenly by optimising the geometric shape and providing better lateral and longitudinal resistance [107,108].It should be noted that producing sleepers with complex shapes using a traditional moulding method is difficult; prototype sleepers with complex shapes for research purposes can be fabricated easily using 3D printing process.

Supporting Layers
Ballasted beds are the assembly of crushed rock particles with specific particle size distribution, which differs according to the designed railway line conditions.The ballast particles move, rotate and interact with each other under dynamic train loads and then deteriorate.The maintenance of a ballasted track is important for keeping ballasted track with enough stiffness and elasticity.One difficulty of ballasted tracks is to detect the ballasted bed conditions.Because the ballasted bed is composed of small discrete particles with a porosity of 0.2-0.4,it is hard to evaluate the mechanical behaviour of the ballasted layer and access the dynamic reactions of ballast particles.References [110][111][112][113] used 3D printing technology to fabricate several smart ballast rocks to record ballast particle movement (e.g., acceleration and rotation) in field tests.The geometric shapes of smart rocks were obtained by mimicking an actual ballast particle shape, and the removable sensor was installed in the 3D printed rock, as shown in Figure 12.Also, ref. [114] adopted similar SmartRocks to evaluate the ballast condition.Reference [115] 3D printed ballast particles with/without predefined fissure using photosensitive resin material and gypsum powder.Their time-dependent behaviours were compared with limestone ballast particles.The 3D scanning approach was adopted to obtain the geometric shape of real limestone ballast particles.References [116,117] tried using 3D printing technology to fabricate the sample with similar mechanical behaviour to real rocks.The results indicate that 3D printed samples have good mechanical behaviour, but the 3D printing technology still needs to be improved [116][117][118][119].

Supporting Layers
Ballasted beds are the assembly of crushed rock particles with specific particle size distribution, which differs according to the designed railway line conditions.The ballast particles move, rotate and interact with each other under dynamic train loads and then deteriorate.The maintenance of a ballasted track is important for keeping ballasted track with enough stiffness and elasticity.One difficulty of ballasted tracks is to detect the ballasted bed conditions.Because the ballasted bed is composed of small discrete particles with a porosity of 0.2-0.4,it is hard to evaluate the mechanical behaviour of the ballasted layer and access the dynamic reactions of ballast particles.References [110][111][112][113] used 3D printing technology to fabricate several smart ballast rocks to record ballast particle movement (e.g., acceleration and rotation) in field tests.The geometric shapes of smart rocks were obtained by mimicking an actual ballast particle shape, and the removable sensor was installed in the 3D printed rock, as shown in Figure 12.Also, ref. [114] adopted similar SmartRocks to evaluate the ballast condition.Reference [115] 3D printed ballast particles with/without predefined fissure using photosensitive resin material and gypsum powder.Their time-dependent behaviours were compared with limestone ballast particles.The 3D scanning approach was adopted to obtain the geometric shape of real limestone ballast particles.References [116,117] tried using 3D printing technology to fabricate the sample with similar mechanical behaviour to real rocks.The results indicate that 3D printed samples have good mechanical behaviour, but the 3D printing technology still needs to be improved [116][117][118][119]. A number of railway track slabs, including sleepers on slab systems, embedded sleeper systems and direct systems, have been developed in the past few decades [5].Most ballast-less track slabs are fabricated from concrete, CA mortar fillings and steel tendons.Many railway companies (e.g., SATEBA in Europe and Stabirail in Belgium) have been developed advanced railway track slabs with optimized geometry for high-speed and heavy-haul railway lines; 3D printing concrete technologies have already been adopted in bridges, civil buildings and art architectures [121,122], which strengthens the railway industry's desire for 3D printed concrete infrastructures.The British HS2 claimed to launch the 'Printfra-structure' project in 2022, which will 3D print concrete slab onsite [123].The concrete materials will be enhanced with graphene for replacing steel tendons.This approach can decrease carbon emission and concrete usage [123].

Special Components
Railway track infrastructure includes some special components for addressing particular issues on sites, such as geogrids and geocells for increasing ballast interaction, retaining walls for constraining ballast movement, sound barriers for noise reduction, and sand guard walls for blocking sands.The 3D printing technologies have been utilized in geogrids, geocells and sound barriers.A number of railway track slabs, including sleepers on slab systems, embedded sleeper systems and direct systems, have been developed in the past few decades [5].Most ballastless track slabs are fabricated from concrete, CA mortar fillings and steel tendons.Many railway companies (e.g., SATEBA in Europe and Stabirail in Belgium) have been developed advanced railway track slabs with optimized geometry for high-speed and heavy-haul railway lines; 3D printing concrete technologies have already been adopted in bridges, civil buildings and art architectures [121,122], which strengthens the railway industry's desire for 3D printed concrete infrastructures.The British HS2 claimed to launch the 'Printfrastructure' project in 2022, which will 3D print concrete slab onsite [123].The concrete materials will be enhanced with graphene for replacing steel tendons.This approach can decrease carbon emission and concrete usage [123].

Special Components
Railway track infrastructure includes some special components for addressing particular issues on sites, such as geogrids and geocells for increasing ballast interaction, retaining walls for constraining ballast movement, sound barriers for noise reduction, and sand guard walls for blocking sands.The 3D printing technologies have been utilized in geogrids, geocells and sound barriers.
A geogrid and geocells are used to increase the cohesion of granular materials.Reference [124] 3D printed two types of 1:10 mode geogrids, as shown in Figure 13a.The 3D-printed models have lower yielding strain (5% to 6%) than full-scale geogrids (10%), which indicates the printed geogrids need to be enhanced.Reference [125] printed a bioinspired honeycomb geogrid for soil reinforcement, as shown in Figure 13b.Ref. [126] 3D printed geocells, as shown in Figure 13c, and optimized the integral node shape.Reference [127] printed a smart geogrid, as shown in Figure 13d.The geogrid is equipped with fibre Bragg grating, which enables real-time accurate strain detection.Reference [128] printed a three-dimensional geogrid, as shown in Figure 13e, and tested the shearing performance.These applications can also be used in ballast layers for increasing the ballast biting effects.
proved.The 3D printing technology can help produce sound barriers with complex geometric properties.A low-height noise barrier for railway systems was reported in [132].The size of the unit cell is present in Figure 14a.The unit can be printed in factories and assembled on site.The approach can reduce more than 50% carbon emission [132].Reference [133] designed a 3D printable ultrathin low-frequency sound absorbing panel, as shown in Figure 14b.Reference [134] designed, 3D printed two types of sound barriers with coiled and rigid unit cells, as shown in Figure 14c, and tested the noise reduction performance.As for 3D printed concrete noise barriers, several acoustic walls, as shown in Figure 14d-f, were built by WINSUN Ltd. in Suzhou, China.These noise barriers can reduce noise by a total of 30 decibels.The 3D printing sound barrier technology is well-developed for adoption in railway noise barriers.
With the development of ultra-high-speed and heavy-haul railway lines, noise has become much more severe and affects the normal life of residents near the railway lines [129].Sound barriers are a solution for reducing the noise caused by moving railway trains [130,131].By optimizing the barrier shape, the noise reduction effect can be improved.The 3D printing technology can help produce sound barriers with complex geometric properties.A lowheight noise barrier for railway systems was reported in [132].The size of the unit cell is present in Figure 14a.The unit can be printed in factories and assembled on site.The approach can reduce more than 50% carbon emission [132].Reference [133] designed a 3D printable ultrathin low-frequency sound absorbing panel, as shown in Figure 14b.Reference [134] designed, 3D printed two types of sound barriers with coiled and rigid unit cells, as shown in Figure 14c, and tested the noise reduction performance.As for 3D printed concrete noise barriers, several acoustic walls, as shown in Figure 14d-f, were built by WINSUN Ltd. in Suzhou, China.These noise barriers can reduce noise by a total of 30 decibels.The 3D printing sound barrier technology is well-developed for adoption in railway noise barriers.

Challenges
Even though additive manufacturing technologies have many advantages and have been applied in some civil buildings and lightweight structures for aerospace fields, the application of 3D printing in railway engineering is still in the explosion stage.This is due to the limitations as follows.

Reliability
The materials used for 3D printing can be classified into three main types, including polymer materials (e.g., photosensitive resin, thermoplastic polymer and hydrogels), metallic materials(e.g., Fe-based, Ti-based, Co-based, Cu-based alloys and etc.) and ceramic materials(e.g., cement materials, clay materials and silicate glass materials) [135,136].Even though those materials can be 3D printed in complex shapes by different additive manufacturing processes, the mechanical properties of many 3D printed samples are usually less than casting samples due to the error in the 3D printing process [137].Conventional railway track infrastructure is of simple geometric shapes and can be produced easily with well-developed conventional railway construction methods.
Moreover, many additive manufacturing technology processes include heating(melting) and cooling processes, which result in uneven deformation and residual stress inside the 3D printed samples.Other printing processes may also produce defects in 3D printed samples, such as small voids (bubbles) and microcracks.These defects will decrease the strength and life cycle of printed samples.The mechanical performance of 3D printed samples is greatly affected by printing speed, printing orientation, printing temperature, etc. [138].However, there is no specific standards for 3D printing samples to ensure the 3D printed sample quality.This uncertainty of 3D printed samples hinders the use of 3D printing technology in railway engineering fields.Many 3D printed samples have rough surfaces due to the poorer accuracy than casting mould methods [135].When additive manufacturing technology is adopted to produce an enhanced layer on the top of rails, grinding and polishing processes are compulsory for making a smooth surface.These processes are time-consuming.

Challenges
Even though additive manufacturing technologies have many advantages and have been applied in some civil buildings and lightweight structures for aerospace fields, the application of 3D printing in railway engineering is still in the explosion stage.This is due to the limitations as follows.

Reliability
The materials used for 3D printing can be classified into three main types, including polymer materials (e.g., photosensitive resin, thermoplastic polymer and hydrogels), metallic materials(e.g., Fe-based, Ti-based, Co-based, Cu-based alloys and etc.) and ceramic materials(e.g., cement materials, clay materials and silicate glass materials) [135,136].Even though those materials can be 3D printed in complex shapes by different additive manufacturing processes, the mechanical properties of many 3D printed samples are usually less than casting samples due to the error in the 3D printing process [137].Conventional railway track infrastructure is of simple geometric shapes and can be produced easily with well-developed conventional railway construction methods.
Moreover, many additive manufacturing technology processes include heating(melting) and cooling processes, which result in uneven deformation and residual stress inside the 3D printed samples.Other printing processes may also produce defects in 3D printed samples, such as small voids (bubbles) and microcracks.These defects will decrease the strength and life cycle of printed samples.The mechanical performance of 3D printed samples is greatly affected by printing speed, printing orientation, printing temperature, etc. [138].However, there is no specific standards for 3D printing samples to ensure the 3D printed sample quality.This uncertainty of 3D printed samples hinders the use of 3D printing technology in railway engineering fields.Many 3D printed samples have rough surfaces due to the poorer accuracy than casting mould methods [135].When additive manufacturing technology is adopted to produce an enhanced layer on the top of rails, grinding and polishing processes are compulsory for making a smooth surface.These processes are time-consuming.
Most importantly, the railway track infrastructure is required to have good fatigue performance under long-term moving train loads [139].Most 3D printed samples are tested under static and quasi-static loads; the behaviours of 3D printed samples under dynamic loads and long-term loads have not been fully understood yet, which may result in the risk of failure of the railway system under long-term service.
In general, the risk in reliability of 3D printing components in railway engineering is caused by poor raw material properties and 3D printing defects.The development of 3D printable materials, technologies and standards will gradually solve the reliability problems of 3D printing in railway track infrastructures.

Size and Space Limitation
The size limitations of different 3D printing technologies are different.The maximum size of 3D printed metallic, and polymer samples are usually less than 1 m, which is limited by the size of 3D printing machines.The length of railway sleepers is 2.6 m, and the width of ballast beds and slab tracks are more than 2.6 m [102].It is not easy to use 3D printing to manufacture all-in-one large-size composite sleepers or track supporting.
3D printing concrete technology allows large size 3D printed constructions, such as 3D printed civil buildings and bridges, but the printing equipment (e.g., robotic arm and removable tracks) needs to be put on-site [140,141].The land areas of 3D printed houses and bridges are in the order of hundreds of square meters, and the construction site is mostly located in downtown areas on plate ground.Unlike these infrastructures, the length of railway lines is much greater, and much railway infrastructure is located on mountain tunnels, underground tunnels, bridges, and in open suburbs.The field conditions make the preparation and placement of 3D printing machines difficult.

Cost
The qualitative comparison of the average cost of fabricating products between 3D printing technology and conventional manufacturing technology is present in Figure 15.The average cost of products fabricated using conventional subtractive manufacturing methods (e.g., CNC) increases when the product complexity increases or when the product quantity decreases.This is because manufacturing products with complex shapes by subtractive technology needs complicated processes (e.g., cutting, drilling and milling).The average cost of 3D printing technologies is not sensitive to product complexity and quantity.Thus, 3D printing technologies show excellent prospects for a small quantity of complex products.However, the geometric shape of railway track infrastructure components is simple, and the demand for railway components for constructing a railway line is massive.This means that using conventional subtractive manufacturing technologies to build railway infrastructures is cheaper than 3D printing technologies, which hinders 3D printing in railway infrastructures.

Perspectives
Based on the previous study, the highlight findings and the knowledge gap of 3D printing application in railway track infrastructures can be described well in Table 2.

Rails
3D printing technologies can be used to repair and enhance rails by generating an enhanced layer on the railhead area.The hardness and fatigue performance can be improved.
The failure modes of 3D printed composite rails will change when the fiber contents changes.
No study uses 3D printing technologies to fabricate a full rail with metallic materials.Also, the mechanical properties of 3D printed composite rails are poor.The 3D printing technologies about metallic materials still need to be improved.The future study can produce a full rail with metals and improve the mechanical properties of 3D printed composite rail by adding different materials with high strength.

Fasteners
3D printing can reduce the fabricating time of steel plates by mould optimization.3D printing allows the production of structures with complex geometric shapes.
There is no fastener component directly fabricated using 3D printing.The clips and the rail pads can be designed and produced with smart shape and high damping porous structures such as honeycomb, lattice and TPMS structures.

Sleepers (bearers)
Composite sleepers (bearers) can be produced with multiply layers using 3D printing technologies.The shape of sleepers(bearers) can be optimised for saving materials and improving track performance.
The research about combining different materials in composite sleepers (bearers) is limited, and the shapes of sleepers (bearers) are simple.Future research can try 3D printed sleepers (bearers) with multi-materials, multi-layers and optimised shapes.
Supporting layers 3D printed smart rocks with internal sensors can help detect the dynamic re-There are limited studies about designing 3D printable one-in-all track support.The re-

Perspectives
Based on the previous study, the highlight findings and the knowledge gap of 3D printing application in railway track infrastructures can be described well in Table 2.

Rails
3D printing technologies can be used to repair and enhance rails by generating an enhanced layer on the railhead area.The hardness and fatigue performance can be improved.The failure modes of 3D printed composite rails will change when the fiber contents changes.
No study uses 3D printing technologies to fabricate a full rail with metallic materials.Also, the mechanical properties of 3D printed composite rails are poor.The 3D printing technologies about metallic materials still need to be improved.The future study can produce a full rail with metals and improve the mechanical properties of 3D printed composite rail by adding different materials with high strength.

Fasteners
3D printing can reduce the fabricating time of steel plates by mould optimization.3D printing allows the production of structures with complex geometric shapes.
There is no fastener component directly fabricated using 3D printing.The clips and the rail pads can be designed and produced with smart shape and high damping porous structures such as honeycomb, lattice and TPMS structures.

Sleepers (bearers)
Composite sleepers (bearers) can be produced with multiply layers using 3D printing technologies.The shape of sleepers(bearers) can be optimised for saving materials and improving track performance.
The research about combining different materials in composite sleepers (bearers) is limited, and the shapes of sleepers (bearers) are simple.Future research can try 3D printed sleepers (bearers) with multi-materials, multi-layers and optimised shapes.

Supporting layers
3D printed smart rocks with internal sensors can help detect the dynamic responses of ballast particles in the ballasted track on sites.3D printing concrete technologies can be used to construct concrete slabs.
There are limited studies about designing 3D printable one-in-all track support.The research about 3D printed track supports with smart structures can be carried out.

Special components
3D printing technologies have already been used to produce concrete sound-proof walls, composite geogrids, and geocells.
The studies about the performance of 3D printed sound-proof walls and geogrids/geocells under actual railway loads are scarce.Future studies can focus on the dynamic responses of 3D printed components under railway loads.
Based on the present research of 3D printing in railway engineering and the pros and cons of additive manufacturing technology, the prospects of 3DP technologies in railway engineering are concluded as the following aspects: 1.
3D printing technologies can help optimize and construct small components of railway infrastructures, e.g., fastener components (clips, screw spikes and rail pads), geogrids and geocells.2.
The rail can be enhanced by additive manufacturing technologies.The approach can help lengthen the life cycle of railway turnout systems by increasing the fatigue behaviour of rails (including stock rails, closure rails, wing rails and guardrails).With the development of new printable metallic and polymer materials, the rails can be fabricated with good mechanical properties using 3D printing technologies.3.
3D printing technologies have the potentials to be used in fabricating composite sleepers (bearers) with multi-layers and producing sleepers with complex shapes.4.
3D printed concrete technologies are well-developed and can be used to construct sound barriers in railway engineering.3D printing concrete slab track is being prepared in the UK. 5.
Smart track detection methods can be developed using 3D printing technologies help to detect the track dynamic response in real-time.6.
The 3D printing technologies enable the fabrication of an ultra-high-speed tube track system (also called hyperloop), which allows the train to speed up to 1200 km/h.Figure 16 indicates an initial conceptual design of the prototype hyperloop railway structure.Moreover, all-in-one track supports can be designed with a gradient density in accordance with the stress distribution inside the railway track support.The track stiffness is designable by changing the porosity and structural form of the all-in-one structures.The all-in-one track supports with gradient porosity will have a gradient track stiffness, which can help solve the problem of discontinuous stiffness in the transition areas.
Special components 3D printing technologies have already been used to produce concrete sound-proof walls, composite geogrids, and geocells.
printed sound-proof walls and geogrids/geocells under actual railway loads are scarce.Future studies can focus on the dynamic responses of 3D printed components under railway loads.
Based on the present research of 3D printing in railway engineering and the pros and cons of additive manufacturing technology, the prospects of 3DP technologies in railway engineering are concluded as the following aspects: 1. 3D printing technologies can help optimize and construct small components of railway infrastructures, e.g., fastener components (clips, screw spikes and rail pads), geogrids and geocells.2. The rail can be enhanced by additive manufacturing technologies.The approach can help lengthen the life cycle of railway turnout systems by increasing the fatigue behaviour of rails (including stock rails, closure rails, wing rails and guardrails).
With the development of new printable metallic and polymer materials, the rails can be fabricated with good mechanical properties using 3D printing technologies.3. 3D printing technologies have the potentials to be used in fabricating composite sleepers (bearers) with multi-layers and producing sleepers with complex shapes.4. 3D printed concrete technologies are well-developed and can be used to construct sound barriers in railway engineering.3D printing concrete slab track is being prepared in the UK. 5. Smart track detection methods can be developed using 3D printing technologies help to detect the track dynamic response in real-time.6.The 3D printing technologies enable the fabrication of an ultra-high-speed tube track system (also called hyperloop), which allows the train to speed up to 1200 km/h.Figure 16 indicates an initial conceptual design of the prototype hyperloop railway structure.Moreover, all-in-one track supports can be designed with a gradient density in accordance with the stress distribution inside the railway track support.The track stiffness is designable by changing the porosity and structural form of the all-in-one structures.The all-in-one track supports with gradient porosity will have a gradient track stiffness, which can help solve the problem of discontinuous stiffness in the transition areas.

Conclusions
This paper presents an overview of 3D printing application and research in railway track infrastructures and the challenges and perspectives of 3D printed railway tracks.The main benefits of additive manufacturing technologies are fabricating complex objects, saving materials, reducing carbon emission, and shortening the product production cycle.The recent research and application of additive manufacturing (3DP) technologies in railway track infrastructures have been reviewed.Presently, the 3DP in railway infrastructures is still in the exploration stage; only the laser melting methods for enhancing rail and wheel surfaces are well developed.This is due to the poor reliability and other limitations of 3D printing technologies for railway engineering.However, 3D printing shows excellent prospects in building all-in-one track infrastructures and designing transition zone supports.In the future, with the development of 3D printing materials, technologies and standards, additive manufacturing will be adopted for building smart railway track infrastructures.This research can promote the research and application of 3D printing in railway engineering.

Figure 5 .
Figure 5. Prototyping rail for micro-people movers using additive manufacturing: (a) 3D printed composite rail; (b) compression and flexural tests.Both have been reproduced from [73].

Figure 5 .
Figure 5. Prototyping rail for micro-people movers using additive manufacturing: (a) 3D printed composite rail; (b) compression and flexural tests.Both have been reproduced from [73].

Figure 6 .
Figure 6.The change of railway fastener systems.

Figure 15 .
Figure 15.Qualitative comparison of costs between 3D printing technology and conventional manufacturing technology.(a) cost of the level of product complexity; (b) cost of the product quantity.

Figure 15 .
Figure 15.Qualitative comparison of costs between 3D printing technology and conventional manufacturing technology.(a) cost of the level of product complexity; (b) cost of the product quantity.

Table 1 .
Materials used in laser cladding repaired and enhanced rail.

Table 2 .
Summary of 3D printing application and research in railway track infrastructure.

Table 2 .
Summary of 3D printing application and research in railway track infrastructure.