Before the operation of newly constructed railways, tamping and stabilizing machines should be used to improve the quality of ballast beds. With the expansion of the railway network and increase of speeds and axle loads, higher quality and efficiency for tamping and stabilizing operation are required. However, previous studies did not involve the effects and parameters of three-sleeper tamping and stabilizing operation under complex working conditions. In the paper, the effect of a three-sleeper tamping and stabilizing machine on the ballast bed state has been studied by performing field experiments. The effect of important factors, including tamping modes, stabilizing frequency, and track lifting amount, are discussed in detail. The results show that the tamping operation on newly constructed railways causes a reduction of the lateral resistance by 56.5 % and a reduction of lateral resistance work by 64.9 %. After the stabilizing operation, the lateral resistance and lateral resistance work are increased by 168.6 % and 209.8 %, respectively. The tamping and stabilizing operation can significantly increase the support stiffness of ballast beds, which meets the requirements of train operation. Meanwhile, 2X tamping mode is more beneficial to improve ballast resistance. Besides, it is reasonable for a stabilizing frequency of 25 Hz to be used for newly constructed railways. The track lifting amount also has a large effect on the ballast bed quality, and it is recommended to keep the lift amount in the range of 20 mm ∼ 30 mm to achieve a better tamping quality.

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Turnout is the key component of the railway tracks for trains to change direction, which is vital to operational safety and passenger comfort. Therefore, it is of great importance to perform a scientific and reasonable tamping operation for turnout areas. In this paper, based on the commercial software EDEM and RECURDYN, a coupled simulation model of the large machine tamping device-rail-sleeper-ballast bed in the turnout area is jointly established, and the correctness of the model is verified by the test results of the lateral resistance of the ballast bed. The influence of tamping operation on the macro-and micromechanical properties of ballast bed at the switching part of railway turnout areas is studied and recommendations for the optimization of tamping operation are proposed. The results show that in the squeezing stage, strong force chains are distributed concentrately under sleepers, where the distribution range is approximately elliptical with a depth of 150 mm. After tamping, only the 200-mm ballast under the sleepers is compacted, where the compactness is increased by 5.9%. On the contrary, the compactness of the ballast in the sleeper crib is reduced by 27.4%, which is the weakest part. To ensure favorable tamping quality, the tamping sequence at the switching part of railway turnout areas is suggested to be conducted in order of first through track and then diverging track.

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The sand intrusion in railway tracks in sandy regions can significantly change the mechanical behaviour of tracks and thus threaten the safety of train operation. This paper presents substantial field tests on both sandy and clean railway tracks to study the effect of sand intrusion on the longitudinal resistance of ballast bed and the vibration behaviour of track structures. After that, a 3D multi-scale the discrete element model is developed to study the micro-contact between ballast particles and the vibration behaviour of sandy tracks during train passing in detail. Also, the effect of train speeds and axle loads on the mechanical behaviour of sandy tracks is discussed. The results show that the sand intrusion increases the vibration acceleration amplitude of rail and sleeper by 11.3% and 50.3%, while ballast bed decreases by 44.9%. Besides, the sand intrusion significantly changes the energy distribution in the track, wherein the frequencies of the highest energy of rail and sleeper are increased while that of the ballast bed is decreased. The parametric study shows the high train speed can cause the increase in overall acceleration of the ballast bed and high axle load can cause an increase in the micro-contact forces between ballast particles, diffusion angle of the contact force chain, displacements of ballast particles, acceleration of ballast particles, and sleeper displacements.

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The bonding contact presents complex modes in polyurethane-mixed ballast. The commonly used parallel bond model is revised and four different contact models are developed including Ballast-Ballast Contact Bonding, Ballast-Ballast Noncontact Bonding, Ballast-Sleeper Contact Bonding, and Ballast-Sleeper Noncontact Bonding. The mechanical behaviour and energy evolution of polyurethane-mixed ballast with various amounts of glue are studied from the macro and mesoscopic properties. Results show that the elastic strain energy has always been the main form in polyurethane-mixed bed, followed by viscous strain energy, frictional energy, and damping energy. Compared with the common ballast bed, there are more contacts in polyurethane-mixed ballast bed and, when more glue is used, the amount of contacts is further increased while the maximum contact force is reduced. After bonding, the amount of contacts is significantly increased and all forms of energy become more evenly distributed at different surfaces of the sleeper. The kinetic energy of polyurethane-mixed ballast fluctuates with smaller amplitude and convergences more quickly under cyclic loading, which is reflected in the macroscopic aspect that the settlement of polyurethane-mixed ballast bed is relatively small and can be fast completed.

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The lateral resistance of ballast bed is an important parameter to prevent track expansion and maintain track stability. The invasion of sand particles can cause the change of lateral resistance of ballast bed and affect the stability of track structure, but little attention has been paid to the change characteristics of nonlinear lateral resistance of sandy ballast bed. In this paper, the field tests on the lateral resistance of windblown sand ballast bed were carried out to establish a multiscale three-dimensional discrete element model of sleeper-ballast bed. A systematic analysis on the evolution of lateral resistance, resistance to lateral deformation, micro-contact characteristics and lateral stability of ballast bed is performed. The results show that sand intrusion can increase the lateral resistance of ballast bed, which is approximately 40 % higher than that of clean ballast bed. In nonlinear strengthening stage and yield stage, the enhancement effect of sand particles on the lateral resistance of ballast bed is relatively weaker in comparison with the linear growth stage. With the increase in sand intrusion depth, the lateral resistance and resistance work of ballast bed both gradually go up, and the contribution of ballast shoulder to lateral resistance tends to play a leading role. Sand intrusion can increase the lateral stiffness of ballast bed and reduce the elasticity of track structure. Therefore, the maintenance operation should be carried out in time for the section with severe sandstorm.

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In order to study the interaction between various fouling particles and ballast, a multi-layer and multi-scale discrete element model (DEM) including the sleeper, ballast bed and the surface layer of subgrade was developed. Two typical fouling particles, the hard particles (sand) and soft ones (coal fines), are considered. A support stiffness test of the ballast bed under various fouling conditions was conducted to calibrate the microscopic parameters of the contact model. With the model, the influence of fouling particles on the mechanical behavior and deformation of the ballast bed was analyzed from macro and micro perspectives. The results show that the increase in the strength of the fouling particles enlarges the stiffness of the ballast bed. Hard particles increase the uniformity coefficient of the contact force bond γ of ballast by 50.4%. Fouling particles increase the average stress in the subgrade, soft particles by 2 kPa and hard particles by 1 kPa. Hard particles can reduce the elasticity, plastic deformation and energy dissipation in the track structure. As the fouling particle changes from hard to soft, the proportion of the settlement in ballast bed increases to 40.5% and surface layer of swbgrade settlement decreases to 59.5%. Thus, the influence of fouling particles should be considered carefully in railway design and maintenance.@en

Due to the limitation of the geological environment, many railways around the world are built in desert areas. The intrusion of sand particles can increase the stiffness of ballast beds and cause corrosion and fracture of fasteners, turnout blockage, etc., even affecting the service safety of railways. In the paper, based on the results of field and laboratory tests, a three-dimensional discrete element model of the sandy ballast bed is developed to study the effect of sand intrusion on the load sharing ratio of sleepers, contact forces between ballast particles, displacements of ballast particles, and energy dissipation in ballast beds. The results show that the axle load in sandy tracks is mainly shared by three sleepers under the single-axle load and six sleepers under the dual-axle load. The maximum contact force under the sleeper in the sandy ballast bed is about 80% larger than that in the clean ballast bed and its normal and tangential contact forces are also larger than that in the clean ballast bed. The ballast particles at the ballast shoulder of the clean ballast bed move downward in the direction of less than 45°, while those in the sandy ballast bed move outward at the direction of 90°. The sand intrusion inhibits the movement of ballast particles and may cause a rheological phenomenon of ballast particles at the ballast shoulder. The intrusion of sand particles mainly affects the elastic strain energy and damping energy in ballast beds but has less effect on friction energy and kinetic energy.@en

Transition zones between railway tracks and bridge decks can cause higher dynamic impacts. A solution is smoothly changing the track stiffness by gradually mixing steel furnace slag into the stone ballast. A nominated bridge transition zone is divided into 5 blocks of 7 meters long, with the mixing percentages of 0%, 25%, 50%, 75% and 100%. The mechanical behaviors of furnace slag-ballast combinations (FS-BCs) were studied using experiments of shear strength test, Los Angles abrasion index and plate load test. Furthermore, the dynamic behavior of bridge transition zone with FS-BCs blocks was investigated using a field validated FEM model. Results show that the 100%, 75%, 50% and 25% furnace slag by weight of ballast can increase the shear strength and ballast layer bending modulus by 13%, 12%, 9% and 7% at speed of 300 km/h compared with those of the stone ballast. The FEM study shows that rail deflections are reduced about 20%, 14%, 21% and 16% at speed of 300 km/h corresponding to 100%, 75%, 50% and 25% FS-BCs and accelerations are significantly reduced as well as increasing FS content of each block in bridge transition zone so that a smooth bridge transition zone can be achieved.

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To ensure railway operations safe, track geometry parameters, e.g., track gauge, are usually inspected using track geometry cars. The measurement frequency of track geometry cars is low (twice per year) due to high operational costs and track possession. An innovative way to perform track inspection at high frequency and affordable cost is using mobile track inspection systems, which can be easily mounted on passenger or freight trains. Besides track geometry, it also creates a digital copy of railway corridors providing asset managers with the ability to make fully informed decisions on track assets. Differently, the collectors of mobile systems are further away from the axle than track geometry cars, which are regarded as unloaded and loaded measurement respectively. This difference may lead to a discrepancy in measurement results. This paper studies the difference between loaded and unloaded measurements, using experimental and numerical methods. In the experimental research, a section of track was measured using both systems. The track longitudinal level measured using unloaded and loaded methods were compared, and the discrepancy reported. It was found that although the measuring distance can cause discrepancies, the unloaded measurement method still meets the measurement requirement. The largest discrepancies are in track transition zones, which is explained using the numerical method. After that, a case study using the unloaded measurement method is presented, wherein a section of track has been measured every month. The results show the advantages of frequent measurements in track inspections and the potential applications of unloaded track inspections.

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Performance evaluation and maintenance planning are gaining importance with ageing rail infrastructure and increasing demand on track safety and continuous availability. The discrete/point railway assets (e.g. bridges, level crossings) together with extended track sections constitute the main railway network infrastructure. The former has important implications in train safety, riding comfort and operating expenditures due to local intensified degradation and plays a role in effective network capacity due to their large quantity. The heterogeneity in asset features and operating environment also adds difficulties to efficient maintenance planning of multiple discrete assets. The current review screens the issue to level crossings, as little concern has been engaged to this asset type, and draws together different perspectives related to their maintenance management. The systems thinking approach is integrated and two levels of asset management (i.e. micro- and macro-level) are used to structure the synthesis, which are interdependent and synergistic. Two major approaches, namely, the mechanistic and data-driven modelling are synthesised. Both contribute to the maintenance knowledge and their comparisons are elaborated. Limitations in existing studies are identified and directions for future research are provided, aiming to contribute to a more refined ‘inspection and diagnosis’ process to properly capture the local track issues and move towards system-level maintenance approach for multiple level crossings.@en

Insulated rail joints (IRJs) are widely used in heavy-haul lines, owing to they are safety critical components of track circuit. However, the IRJs breaks the continuity of the rail, making rail more vulnerable to damage under the long-term wheel-rail interaction. In order to study the performance of IRJs, field investigation and tests were carried out at new IRJ and damaged IRJ. At the same time, a 3D coupling model of heavy haul train-IRJ was established for the further analysis. The result shows that in regards to rail-surface regularity, the new IRJ and the damaged IRJ exhibits convex joint and concave joint respectively. IRJs will gradually turn to concave and dipped in the vicinity of end-post under long-term wheel-rail impact. Since there is a vertical rail displacement difference on both sides of the end-post between the two IRJs, causing the wheel-rail interaction has two different impact modes, of which one is downward height difference impact and the other is upward height difference impact. In normal speed, the impact caused by the damaged IRJ is larger than the new IRJ does and it leads to a severer damage to the whole track. The displacement of sleeper at the damaged IRJ is 7.6 times larger than at the new IRJ due to the void between the sleeper and the ballast. The transfer function of displacement from rail to sleeper at the new IRJ is between 0 and 0.4, while the damaged IRJ is between 0.42 and 6.1. The transfer function can be used as a vital index to evaluate the IRJ service state. With the increase of speed, the displacement of rail and sleeper at the two IRJs increases correspondingly. For the sake of decreasing the occurrence of fatigue damage and plastic deformation of rail and plate as much as possible, the maximum depth of concave should be reined within 0.55 mm.

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For a heavy-haul locomotive within a wheel repairing period, wheel polygonal wear with different operating mileages is obtained by field testing. The test results show that the maximum radial runout of the wheel can increase to 0.87 mm and accompany with the typical damage of wheel tread shelling. Taking the wheel polygons as input excitation, the locomotive-track coupled dynamic model is established, which is verified by the comparisons of test and calculated wheelset vertical acceleration in time and frequency domains. The variable wheel-rail friction coefficient is introduced so as to consider the dry and wet rail conditions. The wheel-rail dynamic contact characteristics under the traction and dry-wet rail surface conditions are analysed simultaneously. It is found that the wheel polygon deteriorates the locomotive traction performance and induces the obvious wheel-rail slipping with large tangential stress, especially in wet rail condition. In dry condition, the wheel-rail could contact generally in the adhesion state. But the longitudinal creep forces fluctuate locally with some larger amplitudes closed to the adhesion force, which is mainly attributed to the excitation of serious wheel polygon. Comparing with the results of the newly repaired wheel, the maximum wheel-rail vertical force, longitudinal force, normal stress and tangential stress at the end of wheel repairing period can increase by 55 kN, 28 kN, 240 MPa and 470 MPa in sequence. The wheel-rail slipping and high-stress state in traction condition should be the dominant factors contributing to the wheel damage of tread shelling.

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Recently, implementing steel slag ballast has been proposed as an appropriate material to substitute stone ballast. In this regard, one of the technical concerns is the behavior of steel slag ballast in both time and frequency domains that needs to be assessed, properly. Furthermore, the combination of stone ballast and steel slag is unavoidable in steel slag ballasted tracks during track maintenance concerning the limitation of steel slag resources. Therefore, this paper suggests an optimal stone ballast-steel slag (SB-SS) combination regarding the dynamic behavior of five SB-SS combinations as 0%SS, 25%SS, 50%SS, 75%SS and 100%SS by weight of ballast using a finite element method (FEM) model of a 50-meter test track. Moreover, using elasticity modulus and Moher-coulomb parameters obtained via a series of plate load and shear strength tests for each SB-SS combination turns FEM model to be more close to the real test track results. Experimental results show that adding steel slag particles to stone ballast increases elasticity modulus and friction angle of ballast layer resulting in the improvement of mechanical behavior of railway track. Consequently, the maximum deflections and root mean square (RMS) of accelerations decrease by increasing steel slag content. Analyzing free vibration of ballast layer combinations reveals that damping ratios of 100%SS ballast layer is the maximum value as 0.25 followed by 75%SS, 50%SS, 25%SS and 0%SS combinations. Moreover, the dominant frequencies of each ballast layer combinations determine that 0%SS, 25%SS and 50%SS coincides within the track excitation frequency range made by wheel sets, while 75%SS and 100%SS are out of which. Finally, according to all results, 75%SS ballast layer is proposed as the optimal SB-SS combination.

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High-speed railways adopt continuous welded rail to maintain the smoothness and continuity of the rail surface. However, the welded joint became one of the weakest parts. In order to clear the characteristics and mechanical properties of the new reinforced device, a dynamic three-dimensional vehicle-reinforced device-track coupling model is established. The mechanical characteristics of the track structure under high-speed train load were simulated and analyzed. After installing the new reinforced device, the dynamic response and service life of the track structure are obviously improved compared with the unreinforced rail. When the train speed is 300 km/h, the dynamic bending stress at the bottom of rail is reduced by 26.90%, the vertical and lateral acceleration of the rail are reduced by 42.78% and 21.56%, the vertical and lateral displacement of the rail are reduced by 6.36% and 8.67%, and the theoretical service life of the rail is greatly extended.

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Transition zones in railway tracks are the locations with considerable changes in supporting structures. Because of the difference in the structures or material properties, the differential settlement always present. The structural vibration caused by the hanging sleepers is often observed in transition zones, which leads to significant amplification of dynamic responses between the ballast and sleeper, aggravating the track degradation. In order to explain the high degradation rate of the track in transition zones, a 3D dynamic (explicit) Finite Element model of transition zones has been used. It considers that the rapid compaction of ballast tracks occurs after a few months of operation, while the track on the engineering structure barely settles, resulting in the appearance of hanging sleepers in transition zones. Therefore, the model can study the differential settlement and the stiffness variation between the ballast track and the engineering structure at the same time. Nonlinear contact elements are used to model the interface between sleepers and ballast so that the behaviour of hanging sleepers can be better studied. The results show that the differential settlement plays a more important role in track degradation than the stiffness variation. The dynamic behaviours are different in the embankment-bridge and bridge-embankment transitions. The location that the maximum wheel-rail interaction force appears does not correspond to that of the maximum ballast stress. The results explain the degradation mechanism and provide guidance for maintenance.@en

The subgrade frost heave of high-speed railways has an inevitable effect on track irregularity in severely cold regions. To research the influence of the subgrade frost heave on the dynamic behavior of high-speed railway vehicles, a multi-body dynamics rigid vehicle model and the finite element method flexible track model are coupled using the rigid-flexible coupling dynamics method. The typical frost heave deformation is used as the excitation source, and the effects on dynamic behaviors of the wheel/rail system are analyzed considering various conditions of frost heave, slab track stochastic irregularity, and train speed. The calculation results show that subgrade frost heave has a significant influence on the safety index (the vertical wheel/rail force and the wheel load reduction rate) and the stability index (the vertical acceleration of the carbody and the vertical sperling index) of the vehicle. Both the safety index and the stability index increases linearly as the frost heave amplitude grows and decreases nonlinearly as the frost heave wavelength grows. The safety index is more sensitive to the wavelength, while the stability index is more sensitive to the amplitude. The frost heave region that has an amplitude greater than 15 mm and a wavelength of less than 15 m should be focused on more. Compared to the frost heave deformation, the growth of slab track stochastic irregularity has greater influence on the vertical wheel/rail force. The influence on the other vertical indexes are relatively insignificant. Higher train speeds lead to a worse safety and stability index, and slowing down the speed is an effective way to ensure the safe and stable operation of the high-speed vehicle.@en

Transition zones in railway tracks are the locations with considerable variation in the vertical stiffness of supporting structures. Typically, they are located near engineering structures, such as bridges, culverts, tunnels and level crossings. In such locations, the variation of the vertical stiffness and the differential track settlement result in amplification of the dynamic forces acting on the track. This amplification contributes to the degradation process of ballast and subgrade, ultimately resulting in the increase of maintenance costs. The paper studies a corrective countermeasure that can mitigate the track degradation in transition zones when differential settlement appears. The countermeasure is the adjustable rail fastener and its working principle is to eliminate the gap under hanging sleepers by adjusting the shims (height of the fastener). The adjustable fasteners are first tested on three transition zones, wherein the adjusted heights of fasteners (accumulated voiding) are recorded after the 2-month and 5-month operation. The test results show the adjustable fasteners are effective to mitigate the track degradation in the transition zones. The effect of the adjustable fasteners on the dynamic behaviour of transition zones is analysed using the FE method. The results show that the adjustable fasteners are effective to reduce the amplification of wheel forces, achieve a better stress distribution in ballast, and decrease the normal stresses in rails in transition zones. Parametric studies are also performed to study the applicability of the adjustable fasteners.

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The paper presents a methodology for comprehensive analysis of railway transition zones (e.g. near bridges), including: an advanced measurement technique which uses a DIC-device to measure the dynamic displacements of rails at multiple locations along the track in transition zones; a Finite Element model of transition zones which considers stiffness variation, differential settlement, hanging sleepers, and vehicle dynamics; an iterative procedure to predict the track settlement in transition zones. An application is presented to demonstrate the effectiveness of the methodology. Experimental analysis, Short-term and Long-term numerical analysis, and Design variation analysis are performed on the transition zone.

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Transition zones in railway tracks are locationswith considerable changes in the rail-supporting structure. Typically, they are located near engineering structures, such as bridges, culverts and tunnels. In such locations, severe differential settlements often occur due to the different material properties and structure behavior. Without timely maintenance, the differential settlement may lead to the damage of track components and loss of passenger’s comfort. To ensure the safety of railway operations and reduce the maintenance costs, it is necessary to consecutively monitor the structural health condition of the transition zones in an economical manner and detect the changes at an early stage. However, using the current in situ monitoring of transition zones is hard to achieve this goal, because most in situ techniques (e.g., track-measuring coaches) are labor-consuming and usually not frequently performed (approximately twice a year in the Netherlands). To tackle the limitations of the in situ techniques, a Satellite Synthetic Aperture Radar (InSAR) system is presented in this paper, which provides a potential solution for a consecutive structural health monitoring of transition zones with bi-/tri-weekly data update and mm-level precision. To demonstrate the feasibility of the InSAR system for monitoring transition zones, a transition zone is tested. The results show that the differential settlement in the transition zone and the settlement rate can be observed and detected by the InSAR measurements. Moreover, the InSAR results are cross-validated against measurements obtained using a measuring coach and a Digital Image Correlation (DIC) device. The results of the three measuring techniques show a good correlation, which proves the applicability of InSAR for the structural health monitoring of transition zones in railway track.

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Transition zones in railway tracks are the locations with considerable changes in the vertical support structures. Typically, they are located near engineering structures, such as bridges, culverts, tunnels and level crossings. In such locations, the variation of the vertical stiffness and the differential settlement of the track (when the foundation settles unevenly) result in amplification of the dynamic forces acting on the track. This amplification contributes to the degradation process of ballast and subgrade, ultimately resulting in the deterioration of vertical track geometry (settlement). To analyse and predict the accumulated settlement of the track in transition zones, a methodology using the iterative procedure is proposed. The methodology includes the finite element simulations of the vehicle-track and sleeper-ballast interaction during a train passing a transition zone; and iterative calculations of accumulated track settlement, based on an empirical model for ballast settlement. The simulations are performed using a 3-D dynamic finite element model (explicit integration) of a track transition zone, which accounts for the differential stiffness and the differential settlement of the track. Also, nonlinear contact elements between sleepers and ballast are used. As a result, the model can perform the detailed analysis of the stresses in ballast and accounts for the effects of vehicle dynamics. The model was validated against field measurements. The empirical settlement model describes the two-stage settlement of ballast and the nonlinear relationship between ballast stresses and permanent settlement. The proposed methodology is demonstrated by calculating the track settlement in the transition zone for 60,000 loading cycles (3.5 MGT). The dynamic responses such as ballast stresses are analysed to study the effect of the settlement. The parametric study of the iteration step used in the accumulated settlement procedure has been performed, based on which the optimal step is suggested.

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