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.

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.

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.

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.

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.

With the train speed and axle load increasing, excessive stresses are produced and transmitted to the ballast layer, inducing rapid ballast degradation. To solve this issue, the under sleeper pads (USPs) have been widely applied between sleepers and ballast particles as the elastic layer. In this research, laboratory tests using half-sleeper track were carried out to study the ballast bed performance with or without the USPs under static and cyclic loading. Results show that applying the USPs reduces the track stiffness and can decrease the settlement. However, installing the USPs increases the ballast bed acceleration and the sleeper vertical acceleration. The contact areas of sleeper-ballast with USPs are over 5 times as those without USPs. The USPs assist reducing ballast degradation mainly by avoiding the ballast particle breakage at the sleeper-ballast interface and can increase the stress distribution at the longitudinal direction.

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.

In this study, the application of a retaining wall was proposed as a solution for reducing the lateral displacement of the ballast layer, particularly in sharp curves and bridges. In this regard, a series of single tie push tests were performed on panels with shoulder ballast widths of 300 mm, 400 mm, 500 mm with and without the presence of L-shaped and T-shaped retaining walls. Overall, it was proven that the application of an L-shaped wall led to a 15.8% increase in the lateral resistance, and that T-shaped walls have a higher impact on the stability of the track. A shoulder width of 400 mm was proposed as the optimum width for ballasted tracks with retaining walls.

To simulate ballast performance accurately and efficiently, the input in discrete element models should be carefully selected, including the contact model and applied particle shape. To study the effects of the contact model and applied particle shape on the ballast performance (shear strength and deformation), the direct shear test (DST) model and the large-scale process simulation test (LPST) model were developed on the basis of two types of contact models, namely the rolling resistance linear (RRL) model and the linear contact (LC) model. Particle shapes are differentiated by clumps. A clump is a sphere assembly for one ballast particle. The results show that compared with the typical LC model, the RRL method is more efficient and realistic to predict shear strength results of ballast assemblies in DSTs. In addition, the RRL contact model can also provide accurate vertical and lateral ballast deformation under the cyclic loading in LPSTs.

The rubber-protected ballast (RPB) is made from natural ballast particles and crumb rubber particles. The crumb rubber is shredded waste tires. RPB was chosen to replace the ballast as it has higher resistance to breakage and abrasion. However, the static and dynamic performance of the RPB has not been confirmed yet. Towards this end, experimental tests and numerical simulations were utilized to study the feasibility of RPB application. Direct shear tests (DSTs) were performed and a DST model and three-sleeper track model with the discrete element method (DEM) were built. The shear strength, settlement, displacement, and acceleration of the RPB were studied. The results show that the RPB has the advantage of increasing the force (stress) distribution and that the smaller crumb rubber size was more suitable for replacing the ballast particles.

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.

To mitigate the ballast flight risk in the high-speed railway, this paper presents three new polyurethane bonding schemes which have negligible influence to tamping operations. With the application of these bonding schemes, a series of laboratory tests indicated that the polyurethane-reinforced ballast exhibited much larger lateral resistance than the ordinary ballast by 31% at least. Discrete element simulation results further demonstrated that the polyurethane improved the load-bearing capacity of the ballast at the particle scale through effectively restraining the particle movement. Therefore, the proposed bonding schemes ensure adequate lateral ballast resistance and are effective measures for improving the ballast performance.

Ballast track is the most widely used track for the railway transport, and ballast bed plays a significant role to provide resistances during train operation. Generally, the ballast bed consists of crushed stones. To achieve the mitigation of ballast degradation, the first priority is to describe the degradation development and to study its effect factors. The influence of ballast morphology (particle size and shape) on ballast degradation is examined here using the Los Angeles Abrasion (LAA) test in combination with 3-D image analysis. LAA tests are used to obtain the deteriorated ballast. Then, based on the 3-D images, the changes of ballast particles after the tests were analysed. To quantify the ballast degradation (abrasion and breakage), the Abrasion Depth based on the analysis of 3-D images were proposed, while ballast breakage was estimated using the broken particles ratio. The results have shown that ballast degradation is directly related to the ballast morphology. The proposed image-based procedure can effectively be applied to assess ballast degradation. The results can be used for ballast material standardization, modelling of ballast degradation process and maintenance cycle prediction.