Railway track transition zones present engineering challenges due to their abrupt change in stiffness between structural elements such as embankments, bridges and tunnels affecting track geometry parameters. Although a variety of stiffness-based remedial measures have been widely applied, their implementation can be constrained by high capital cost, operational disruption, and the complexities associated with modifying the substructure. As a result, interventions in practice commonly focus on controlling permanent deformations and differential settlement, particularly related to the development of hanging sleepers. Thus, this study investigates the use of modular self-levelling sleepers (SLS) as a solution. To do so, two concept SLS systems are designed and developed: one employing a granular mechanism (SLS-G), and the other based on a horizontally acting wedge mechanism (SLS-HW). Both variants use the polymeric sleepers and are designed for compatibility with conventional ballasted track systems. Experimental laboratory testing is undertaken, and it is found that the SLS prototypes were able to restore the sleeper-ballast contact for voids up to 40 mm depth, while stress measurements at the interface indicated improved load distribution under the rails. The findings support the proof-of-concept that self-levelling sleepers have the potential to be a modular, low-disruption solution for mitigating track geometry degradation and reducing maintenance requirements at transition zones.

This editorial is referred to the Special Issue (SI) “Smart Condition Assessment of Railway Infrastructure” which aims to bring together the latest research studies, findings, and achievements regarding the smart condition assessment of railway infrastructure to prevent critical failure mechanisms. This SI counted with 20 high quality technical and scientific contributions involving 112 authors of 5 countries.

Differential railway track settlement can result in ballast voids, leading to sleepers that hang from the rail and are no longer supported by the ballast. These hanging sleepers are damage for track component. As a solution, this paper proposes and investigates a new concept sleeper with a wedge-shaped geometry, intended to stimulate the migration of ballast into any voids, thus reducing the occurrence of hanging sleepers. A series of scaled laboratory tests and 2D and 3D discrete element simulations are used to investigate different wedge-shaped geometries. The investigations include the wedge type (single long wedge versus multiple mini-wedges) and the wedge angle (30, 45, 60 degrees). First, the scaled laboratory tests are used to study the performance of different wedge geometries. Next, 3D DEM simulations are performed to analyse the contact forces in the ballast due to different wedge designs. Finally, 2D DEM simulations are performed to study the settlement behaviour. The main conclusions are that a single long wedge is preferable compared to multiple smaller wedges. when the wedge sleeper angle is larger than the ballast's angle of repose, particles have the freedom to migrate into the settlement induced voids. Also, an increased wedge sleeper angle stimulates greater particle migration and thus improves the support correction. However the longer wedge also leads to a decrease in effective ballast height under sleeper which may make retrofitting on existing lines challenging.

The railway ballast layer provides the function of bearing loading, resisting geometry degradation, and drainage. In those related research, the behaviour of ballast assembly can be obtained by laboratory (or in-situ) tests. Limited simulation methods can be used to analyse the behaviour of ballast particles at the mesoscopic level. The numerical simulations based on the Discrete Element Method (DEM) are employed, which treat every ballast particle as a calculation component. However, the efficiency of DEM simulation is very low due to the algorithm and a very large number of elements. This paper analysed the efficiency-related questions of the DEM modelling. The influence of particle shape and contact properties on the force behaviour is studied. Further, an optimised multi-layer ballast track model is introduced based on the most influential ballast areas. In such areas, particles are generated with an irregular shape to ensure the reliability of results, and particles except that area are generated with a rolling resisted ball shape to decrease the number of elements. A series of lateral resistance simulations are conducted to show and validate the accuracy and efficiency of this method in the dimension of the single sleeper section. Results show that this optimised multi-layer model building method largely improves efficiency, and it can provide accurate data.

Railway track transitions are zones where there is an abrupt change in the track-ground structure. They are often the location of rapid track deterioration, which means more frequent track maintenance is needed compared to plain line tracks. With the aim of reducing maintenance, modern transition zone designs use tapered stiffness earthwork profiles to minimise train-track dynamics. However, there has been limited comparison regarding the effect of different tapered profiles on dynamic behaviour. Therefore, this paper's novelty is the investigation of the performance of different earthwork designs in smoothing stiffness transition's considering different types of improvement and also train speed. To do so, first a 3D finite element track model is developed, with support conditions transitioning from an earth embankment onto a concrete bridge. A dynamic moving train load is simulated using a rigid multi-body approach capable of accounting for train-track interaction. The model is used to study the effect of four earthwork solutions with differing stiffness tapers. For each scenario, two different track structure types (ballast and concrete slab) are considered, along with different magnitudes of ground improvement. Lastly, the effects of train speed are explored. It is found tapered earthwork solutions for ballasted tracks show greater dynamic improvement compared to slabs due to their reduced bending stiffness. Further, the more complex improvement geometries such as double trapezoid shapes offer some additional improvement at locations within 3 m of the bridge. However, when considering such tapered stiffness-based earthwork solutions, additional factors such as constructability must also be considered.

Railway ballast performance is dictated by a complex mix of mechanical properties. These effect its performance at the particle level for example in terms of particle degradation, but also at the track system level in terms of settlement and stability. Therefore this paper seeks to develop new understandings of ballast behaviour and identify opportunities for future research directions. First, ballast particle size and size distribution curves are discussed, considering opportunities to improve breakage, settlement and drainage characteristics. Next, particle geometry is discussed, with a focus on form, angularity and surface texture. This is followed by a discussion on the degradation mechanisms of ballast particles and the effect of fouling on permeability. Next, techniques to assess and improve ballast bulk density are discussed, such as ground penetration radar and dynamic track stabilisation. Testing methods for studying ballast are also reviewed, first considering both smaller-scale tests such as direct shear tests and the Los Angeles abrasion test. Then larger-scale laboratory testing is discussed, including large-diameter dynamic triaxial testing and the use of full-scale laboratory tracks. Finally, conclusions are drawn and suggestions for future research directions are given.

Railway ballast is normally made of crushed rocks with grading (particle size distributions). Ballast is inevitably suffering from more rapid degradation. Because ballast keeps undergoing and dissipating most of the train loadings, furthermore, the train speed and freight weight are increasing, causing more intensive loadings to ballast. To prolong ballast service time and reduce ballast maintenance cost, more studies need to be performed on the ballast degradation reduction, ballast inspection, and ballast assessment. Therefore, earlier distinguished studies on these three aspects are introduced, summarized and discussed in this chapter, toward the final goal of providing research gaps, engineering guidance, and maintenance advice.

Crumb rubber (CR) has been proposed to apply in the ballast or sub-ballast layer for ballast degradation mitigation and vibration (noise) reduction. The CR can change the ballast layer stiffness, which can affect the train-track-subgrade dynamic performance and cause travel comfort and safety issues. Towards this, this study aims at confirming 1) how much the CR application can affect the dynamic performance of train and ballast layer; 2) to what extent the CR-ballast layer can distribute the train loadings to reduce subgrade surface stress. To achieve this aim, a whole train-track-subgrade system model was built by coupling multibody dynamics (MD), discrete element method (DEM) and finite difference method (FDM). The MD was used to build the train, including one vehicle body, two bogies and four wheelsets. The DEM was used to build the ballasted track, including rail, sleepers and ballast layer. The FDM was used to build the subgrade. Using the coupled model, the dynamic performance of train and track were studied, including the vehicle body acceleration, wheel-rail force, rail dynamical bending moment, sleeper acceleration, sleeper displacement and ballast acceleration. In addition, the energy dissipation of the ballast bed was also presented. For the subgrade, the subgrade surface acceleration and surface stress were measured and analysed. In the model, different CR size and percentage were considered. Results show that using the CR in ballast layer can increase the accelerations of sleeper, rail and train. But it can decrease the ballast degradation, subgrade surface acceleration and subgrade surface stress. CR helps consume train loading energy, reducing the energy that has to be consumed by ballast friction. Small size CR (8–22.4 mm) has greater influence on dynamic performance of the whole train-track-subgrade system than big size CR (9.5–63 mm). In summary, 10% percentage of CR-ballast mixture is recommended, and for CR size it is difficult to give a recommendation. Small size CR increase ballast acceleration more than big size CR, but small size CR are better at improving sleeper displacement, subgrade stress and ballast bed stress.

Sleeper spacing has been a taboo subject throughout the railway’s history. Safety concerns related to the structural integrity have been the main causes of not addressing this matter. There are no specific and clear recommendations or guidelines in relation to this matter and the distances do not go more than 0.8 m. In order to go beyond this current situation, the following research paper analyses the influence of the spacing between sleepers on the behaviour of ballasted tracks by performing a dynamic simulation with finite elements in two dimensions for different track configurations, different elements, geometries, and separations within the frame of the ODSTRACK project. The variables studied are the vertical displacements, the forces and stresses on the most important elements of the superstructure, as well as the vertical accelerations in the sleepers and the train. The values obtained from the numerical simulations were compared with the maximum permitted values according to the guidelines. To limit this distance to the most restrictive variable among those analysed, it is necessary to make important assumptions, such as the permissible values and effective support contact areas between the sleepers and the ballast. The preliminary analyses carried out shed light on a possible increment of the spacing between sleepers’ axes up to more than 0.8 m. This suggests that important savings in railways construction costs can be achieved, and they will help to develop the next stage of the ODSTRACK project.

Railway track transition zones are areas where there is a sudden change in the track-ground structure. They include changes between ballasted and slab track, bridge approaches, and tunnel entry/exits. They are often the location of rapid track deterioration, and therefore this paper investigates the use of auxiliary rails and soil improvement to minimise train-track-ground dynamic effects. To do so, a 3D finite element model is developed using eight-node solid elements and a perfectly matched layer absorbing boundary condition. A moving train load is simulated using a sprung mass model to represent train-track interaction. After presenting the model, it is validated against field data collected on both a plain line and at a transition zone. Once validated, a sensitivity study is performed into auxiliary rails and soil improvement. It is found that auxiliary rails can improve the dynamic characteristics of the track across the transition, and that more widely spaced auxiliary rails provide greater benefit compared to closely spaced ones. Regarding soil improvement, a large benefit is found, and for the material properties under investigation, the effect of soil stiffening is greater than using auxiliary rails.

In the present paper the mechanical behavior of two types of Kunststof Lankhorst Product (KLP) sleeper, namely low-density polyethylene sleeper (LDPE-16) and high-density polyethylene sleeper (HDPE-25) with 16 mm and 25 mm steel bars diameter, are studied. To this end, the static, dynamic, and longtime static three points bending moment tests are performed. The HDPE-25 and LDPE-16 with six strain gauges are mounted on the steel bars to assess their mechanical responses. Moreover, a Finite Element Method (FEM) model is developed to perform sensitivity analysis on 16 mm HDPE (HDPE-16) and 25 mm LDPE (LDPE-25) based on different steel bar diameters. The results showed that the LDPE-16 steel bar yielded under a load of 30 kN for 4 hours, while HDPE-25 showed significant resistance. Numerical results showed that HDPE-25 is overdesigned and can be replaced by LDPE-25, which is lighter in weight and lower in price. The natural frequencies of HDPE-25 were almost 16%, 19%, 16%, and 33% higher than the three first bending frequencies and first torsion frequency of LDPE-25, respectively. These findings prove the better performance of LDPE-25 in the case of preventing resonance. In addition, the exural modulus of HDPE-25 was almost 42%, 45%, and 65% higher than that of HDPE-16, LDPE-25, and LDPE-16, respectively.

To enhance the stability of continuous welded rail (CWR) tracks, frictional sleepers have been developed. The frictional sleepers are new types of sleepers with grooves on the bottom, and different bottom grooves improve lateral resistances at different magnitudes. In this study, single sleeper push test (SSPT) and its model with discrete element method (DEM) were carried out to confirm how much arrowhead groove frictional (AGF) sleeper increases the lateral resistance of ballasted track. The SSPTs were performed to confirm the lateral resistance results, and also to validate and calibrate the DEM models. With the validated models, the groove factors influencing the lateral resistances were studied, including groove sizes (depth, width), arrowhead groove direction and groove numbers. The reason of lateral resistance improvement was studied at mesoscopic level, including the ballast-sleeper contact numbers and contact force chains. Results show that applying the AGF sleeper is able to improve lateral resistance by 7–24%, and it can provide enough lateral resistance after reducing ballast shoulder width from 500 mm to 300 mm. The AGF sleeper can improve the sleeper-ballast interaction by increasing sleeper-ballast contact number. The study is helpful for frictional sleeper design, further improving track stability.

Ballast rheology is a phenomenon that describes movements of ballast particles due to the discrete nature, which eventually leads to the ballast bed fluid deformation after a long-time service. In most cases, ballast rheology is the main reason of track irregularity that leads to some track defects, e.g., hanging sleeper and mud spots. Therefore, it is significant to confirm the ballast rheology mechanism, which not only benefits for alleviating track defects, prolonging track service and providing safe transportation, but also provides an innovated means for accurately calibrating the discrete element method (DEM) models. Towards this aim, the Particle Image Velocimetry (PIV) is utilised to study ballast rheology through measuring ballast particle displacements in the single sleeper push test (SSPT). The ballast rheology results are compared with those from the DEM SSPT model, through which the DEM model is calibrated. Results show that the PIV is an effective technical means for ballast rheology study and DEM model calibration. This study is helpful for the researchers to build more precise DEM models, further providing theoretical methodology for ballast track construction and innovations.

Lateral and longitudinal resistance of ballasted track are two main indicators for the track stability quantification. Aiming at improving the lateral and longitudinal resistance, nailed sleeper is studied with single sleeper push tests (SSPTs) and discrete element modelling (DEM). The SSPTs were applied to study how much resistance the nailed sleeper can improve, considering different nail lengths (100, 200, 400 mm), and also used to calibrate and validate the DEM models. With the validated DEM models, different simulation conditions were performed and compared to confirm the optimal nail length (100, 200 mm) and nail number (2, 4). Results show that applying nailed sleepers improves the lateral resistance by 53.7% and the longitudinal resistance by 39.2%. 4 nails, compared to 2 nails, can increase lateral and longitudinal resistance by 20.2% and 10.6% (nail length: 100 mm) as well as 37.0% and 33.5% (nail length: 200 mm), respectively.

Railway ballast beds bear cyclic loadings from vehicles and deteriorate due to ballast particle degradation (breakage and abrasion), ballast pockets (subgrade defects), fouling (or contamination) and plastic deformation of the beds. Ballast bed deterioration changes the ballast track geometry, which leads to uncomfortable rides, exacerbates wheel-rail interactions and, most importantly, causes safety issues (e.g., derailment). To align the track geometry, tamping is the most widely used means of filling ballast-sleeper gaps and homogenizing ballast beds. Although many studies have been performed on tamping, some necessary research gaps still need to be addressed. To stress the research gaps, tamping studies are critically reviewed in this paper, and the tamping mechanisms, challenges and proposed solutions are introduced and discussed. This review aims to 1) help researchers discover important research directions related to tamping, 2) propose means for tamping methodology improvement/development, and 3) provide advice for developing novel railway track maintenance.

The lateral stability of ballasted track becomes more important because of the safety requirement under the demand of higher train speed and heavier axle load. To increase the lateral resistance of ballast bed, this paper proposes three types of new sleepers, frictional sleepers. The frictional sleepers are sleepers with different shapes of textures attached at the sleeper bottom. To study the application feasibility of the frictional sleepers, experimental tests (single sleeper pull-out test) and numerical simulation (discrete element modelling) are performed. The lateral resistances of the three types of frictional sleepers are compared with the traditional sleeper based on the experimental test, and the mechanism of the lateral resistance increase is revealed according to the numerical simulation. The results indicate that the frictional sleepers can increase the lateral resistance by 32% (maximum), due to the enhanced interaction between sleeper and ballast particles. More importantly, different types of frictional sleepers have different performances, and the optimal friction sleeper is confirmed. This study is helpful for the further research on sleeper design.

This paper presents the development of a multi-body system (MBS) vehicle-crossing model and its application in the structural health monitoring (SHM) of railway crossings. The vehicle and track configurations in the model were adjusted to best match the real-life situation. By using the measurement results obtained from an instrumented crossing and the simulation results from a finite element (FE) model, the MBS model was validated and verified. The results showed that the main outputs of the MBS model correlated reasonably well with those from both the measurements and the FE model. The MBS and FE models formed the basis of an integrated analysis tool, which can be applied to thoroughly study the performance of railway crossings. As part of the SHM system for railway crossings developed at Delft University of Technology, the MBS model was applied to identify the condition stage of a monitored railway crossing. The numerical results confirmed the highly degraded crossing condition. By using the measured degradation as the input in the MBS model, the primary damage sources were further verified. Through identifying the crossing condition stage and verifying the damage source, necessary and timely maintenance can be planned. These actions will help to avoid crossing failure and unexpected traffic interruptions, which will ultimately lead to sustainable railway infrastructure.