The track structure of a light-rail transit system is designed to pass through a city center or connect with other transportation systems both inside and outside a city. Light-rail transit is a form of urban rail transit using rolling stock similar to a tram, but operating at a higher capacity, speed, and often on an exclusive right-of-way. Urban railway is a type of local rail system providing passenger service within and around an urban area. Light-rail systems are characterized by weaker track conditions than a typical urban railway. The Korean light-rail system is built around a city center that has already been formed, making it difficult to secure large areas of land such as for a vehicle base. The form of the vehicle base is thus constructed as a steep curve; nevertheless, as it is the main track rather than the vehicle base, it is constructed as a ballasted track rather than a slab track. Similarly, because it is a vehicle base, joint rails are applied and not CWR. In fact, Korean light-rail vehicle bases frequently undergo rail joint repairs. Moreover, the track geometries of not only the main track, but also station tracks, subsidiary main tracks, and side tracks often consist of multiple segments with sharp curves owing to a typical lack of available land in the city center. Figure 1
shows the typical damage of a rail joint connected to a joint plate without rail welding. As a typical mechanism of rail joint damage, misalignment occurs during train operation at the end of the two rails in contact with each other and manifests in the form of pressing and breakage in Figure 1
. Rail joints are divided into suspended joints in which the rail joint is positioned between two sleepers and supported joints, which are positioned directly above the sleeper [1
]. The upper photograph in Figure 1
shows a suspended joint and the lower photograph shows a supported joint. In the case of load in the area of the joint, the bending lines of both rails are not continuous, but form an angle [1
]. The wheels have to overcome this angle and, therefore, exert an impact on the accepting rail [1
]. This study measured the typical defects by rail joint type of supported joint. On the basis of this study, even for a vehicle base rather than a main track, we applied the design condition (impact factor) considering the rail joint impact in the design of rail joints for vehicle base installations, with the hope that the new research results for reinforcing rail joints can be applied in the field.
The rail joint is a necessary component in track system. As the rail joints have low rigidity and strength compared with the rail centers, local settlement occurs. Further, the impact of a vehicle is generated to reduce the ride comfort and increase the maintenance. Damage can occur in the wheels and jointed rail ends when a train passes over discontinuity gaps in the rail joints, which can impact the rail, rail fastening system, sleeper, and even the ballast (Figure 1
). This phenomenon represents a disadvantage of the entire track system as it easily subjects the trains to impacts or vibrations, reduces passenger comfort, and requires considerable manpower and expenditure for track and wheel maintenance [2
In terms of rail joint types, most recent railways use continuous welded rails (CWRs), which comprise several long or short rails welded together. In conventional CWRs, fishplates are employed around the rail joint to connect the web of the adjacent rail by bolts, thereby removing the discontinuity gap in the jointed rail and significantly reducing impact and noise during train operation.
A substantial amount of research both in Korea and abroad has focused on improving rail joints in order to mitigate rail joint impacts. Dynamic wheel–rail forces are generated when a vehicle is subjected to dynamic motions, primarily owing to track irregularities and changes in the track geometry. Geometrical track irregularities, unsprung and sprung masses of vehicle, track stiffness, damping variations in track flexibility, wheel flats, and corrugations on wheels and rails generate dynamic forces on the tracks [2
]. For example, Lim et al. conducted an analytical study and proposed a method to limit sleeper displacement in the movable section of newly constructed joints to a set value [4
]. In addition, Zong et al. proposed a dynamic wheel–rail contact impact modeling method for the determination of impact loads. Their results demonstrated that the free edge of the joint gap draws significant residual stress concentration at the top corner of the end of the railhead, resulting in early material failure at the joint [5
]. Mandal and Peach presented limited measurements as analysis and simulations were carried out instead to address mechanical failure, such as failure of the joint bar, joint looseness, and height mismatch of insulated rail joints. From their results, it is known that there is a small reduction in the stress encountered by the rail when joined by bars with an increased moment of inertia [6
]. Pombo and Ambrósio proposed a wheel–rail contact model with small-radius curved tracks [7
], whereas Sugiyama et al. compared the steady-state lateral force of small-radius curved tracks through track tests and simulations [8
]. Wen et al. [9
] used finite element analysis simulations to determine the effects of the impact of wheel–rail contact at the rail joint on the axle load, the effect of train speed on dynamic vertical forces, and the distributions of railhead stress and strain. Sharma and Kumar conducted a dynamic analysis of wheel–rail contact and confirmed that the pressure distribution was affected by the rolling distance under various track curvatures [10
]. In addition, the wheel–rail impact contact of the insulation rail joint has been simulated through numerical analysis [11
]. Choi and Kim studied the impact on the bearings that support the track girder. They performed field measurements and numerical analysis to analyze the characteristics of the structural behavior of a track installed on the Yeongjong Grand Bridge based on the type of bearings and that of the bearings themselves [12
]. Raymond found that hardened tracks have smaller differential settlements and cause a lower track impact effect than softer tracks [13
]. Selig and Waters found that the subgrade is the important component of the substructure, and it affects track stiffness greatly [14
]. Liang and Zhu found the higher deformation and instability of the track structure including the ballast occurring by the reduction of subgrade stiffness [15
]. Thus, the majority of the previous studies have analyzed the behavior of main line tracks or the service life of CWRs, which are characterized by a relatively lower track impact during train operation. However, to enter the tracks of the main line, all trains must travel on tracks that have not been converted to CWRs and that contain many rail joints, which include the vehicle base, station tracks, side tracks, and subsidiary main tracks. Therefore, the importance of specifically analyzing the behavior of rail joints cannot be overlooked. The track impact factor (TIF) is a theoretical index used in the track design, which can vary owing to various complex factors such as the structural characteristics of the track, track support stiffness, characteristics of the vehicle, running speed, rail surface roughness, and track irregularity. In this paper, an experimental study of the TIF is discussed. In the “Field Measurement” section, the dynamic wheel–rail contact impact forces are investigated and discussed with the TIF of the rail joint and CWRs. The field measurements were performed using operational trains on different track types such as CWRs of concrete slab tracks in the main line and rail joints of ballasted tracks in the vehicle base. Using these measurements, the rate of dynamic wheel load fluctuation and the TIF were calculated for the CWR and rail joint sections. Subsequently, the calculated TIF values were analytically validated through a comparison with the measured vertical rail displacement, finite element analysis (FEA), and the designed TIF for rail joints and CWRs. Finally, the TIF measured by field measurement was compared with the result predicted by FEA.