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• Ballast derived from different parent rocks were reinforced with CR. • Degradation of ballast subjected to various impact loads was measured. • The big dataset was employed to train and test the machine learning models. • Parent rock type was the most influential parameter based on ML models. Parent rock strength and crumb rubber modification are two critical mechanical parameters that significantly decide the ballast layer degradation subjected to train dynamic loading. Using machine learning to predict ballast degradation considering these two parameters is helpful for deciding ballasted track maintenance cycle. In the current study, the ballast degradation process data (variables: parent rock types, loading types, ballast gradations and compositions of crumb rubber-ballast mixture) were used to train machine learning models. The drop-weight impact loading tests were performed to simulate different train dynamic loadings. Two well-established machine learning models, i.e., random forest (RF) and support vector regression (SVR) were trained and verified, to more effectively assess the importance of these variables. The results from the validated machine learning models confirm that the parent rock type is the most influential parameter, followed by the loading type (applied stress level), to control and predict the degradation of the ballast-CR mixture. The experimental assessment reveals that although the incorporation of CR suppresses degradation across all characterized rock types, the improvement in performance of the ballast-CR specimen against degradation is more noticeable for high-strength parent rock subjected to a considerable stress level. Meanwhile, this positive influence is also observed for ballast of weaker strength when the applied stress level is low. [ABSTRACT FROM AUTHOR]

• Quantified assessment of individual ballast degradation based on 3D particle images. • Illustration of ballast breakage type and quantified using surface area change. • Improvements in ballast surface texture quantification with 3D particle images. • Reuse of waste rubber in railway engineering (ballast layer). Higher speed, more freight and frequent maintenance increase ballast degradation and reduce the ballast lifespan. To reduce ballast degradation, crumb rubber used as buffering aggregates in ballast bed is relatively unexplored and needs more studies, because using it has the advantage of reusing the waster rubber and absorbing the noises. The effects of crumb rubber (CR) size and percentage on ballast degradation reduction is studied in this paper, and the optimal CR size and percentage are proposed. Three CR size ranges are utilised, i.e., 3 ∼ 5 mm, 10 ∼ 15 mm and 20 ∼ 25 mm, and the percentages are 0, 10, 20 and 30% by weight. Three kinds of ballast material with two size ranges are utilised. The deteriorated ballast particles were generated using Los Angeles Abrasion (LAA) tests, and the ballast degradation was evaluated with the 3D image analysis. The results indicate that ballast abrasion can be alleviated by adding the CR, while the CR has few influences on the ballast breakage. When CR size is close to ballast particle size, the effects of degradation reduction are not obvious. The corner and edge loss are the main types of ballast abrasion, although different ballast materials significantly influence the abrasion type and degree. Most importantly, the image analysis method is proved to have the ability to present ballast degradation process and has great potential for further degradation-related studies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Summary and comparison of ballast standards in different countries. • Promising future ballast technologies are discussed. • Ballast qualification considering different material types and local geologies. The properties of railway ballast material are affected by the local geologies and climatic environments from which the parent rock is sourced. These factors can make it challenging to select the most appropriate material for railway applications. To address this issue, this paper first reviews the means of ballast selection in complex environments across the world. The selection criteria for ballast materials are compared and test methods for ballast quality quantification are summarised. Next, ballast parent rock types and the implications of mining approaches are discussed, before analysing ballast morphology with respect to ballast size and shape. Then ballast petrography is reviewed with a focus on the effect of mineral composition on performance. Finally, some promising future ballast technologies are discussed with a focus on environmental performance. These include recycled ballast, asphaltic materials, steel slag and ballast gluing. The review shows that regarding ballast selection means and criteria, the number and type of quantitative indicators varies greatly between countries. In particular there are divergences in test methods and quantitative indicators for ballast quality considering material types and local geologies. Suggested future research directions are proposed, such as the effect of tamping and dynamic track stabilisation on ballast properties. [ABSTRACT FROM AUTHOR]

• Six diverse railway lines were inspected by the GPR, proving the GPR application feasibility for diverse railway line conditions. • Accuracy of the GPR-based fouling index is correlated with sieving-based fouling index. • By comparing the sensitivity of different indicators, the suitable GPR signal processing means is obtained. • Fouling composition is proved as a decisive factor for GPR-based fouling level evaluation, especially the iron ore. Ballast layer condition should be more regularly and accurately inspected to ensure safe train operation; however, traditional inspection methods cannot sufficiently fulfil this task. This paper presents a method of ground penetrating radar (GPR) application to reflect ballast layer fouling levels under diverse field conditions (annual gross passing load, cleaning and renewal year, fouling composition and transportation type). The results show that the GPR-based inspection method can assess the ballast layer fouling level with a 1–7% difference from the traditional sieving results. Fouling composition (especially metal materials) has a great effect on the GPR signals, thus affecting the inspection accuracy of ballast layer fouling level. Developing diverse GPR-based fouling indicators (by distinguishing different GPR signal features) can improve the GPR inspection applicability to the diverse field conditions. [ABSTRACT FROM AUTHOR]

• Coupled DEM-FDM-MBD model to study train-track-subgrade dynamic responses. • Crumb rubber (or tire-derived aggregates) effects on train-track-subgrade dynamic responses. • Crumb rubber size effects on the train-track-subgrade dynamic responses presented by ballast friction energy dissipation. 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. [ABSTRACT FROM AUTHOR]

• Overview of tamping to ballasted track. • Included plenty of the published studies on tamping to ballast layer. • Presented cutting-edge techniques or methods for ballasted track maintenance. • Explained state-of-art methods for ballasted track inspection. 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. [ABSTRACT FROM AUTHOR]

• Particle Image Velocimetry (PIV) was applied for DEM model calibration on mesoscopic level. • Ballast rheology (movements) of one ballast bed cross section is presented and analysed. • Lateral resistance of ballast bed to sleeper is studied from the perspective of ballast rheology. 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. [ABSTRACT FROM AUTHOR]

• Innovated sleeper shape with texture attached at sleeper bottom. • Comparison of different friction sleeper design. • Discrete element modelling of sleeper-ballast interaction. • A rapid method for discrete element method model construction. 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. [ABSTRACT FROM AUTHOR]

• The performance of recycled and fresh ballast mixture was studied. • The boundary of recycled ballast mixing ratio was found out. • Ballast surface texture was employed in DEM to represent both recycled and fresh ballast. • Recycled ballast could be reinforced by fresh ballast addition. Ballast degradation is frequently observed under cyclic loading, and results in bearing capacity and drainage problem of ballast track. To keep the stability and safety, periodical maintenances are needed, such as cleaning and replacement, which produce a huge amount of wasted ballast. Thus, reusing the deteriorated ballast can become a considerable method for sustainable railway development and environment protection. One applications is adding the cleaned deteriorated ballast (i.e. recycled ballast) into fresh ballast. Furthermore, it is common situation that applying the mixture of fresh and deteriorated ballast during the railway operation. To study the mechanical behaviour of this mixture and find out the criterion weight proportion of the recycled ballast, a series of large direct shear tests were performed with different weight proportions (0%, 10%, 20%, 30%, 40%, and 50%) of recycled ballast mixed into fresh ballast under different normal stresses (50, 100 and 200 kPa). In addition, a numerical simulation based on discrete element method (DEM) was used to illustrate the shear strength, contact forces, coordination numbers and displacements of ballast particles. Results show that the shear strength reduction of the mixture is insignificant, when mixed with less than 30% recycled ballast. With the recycled ballast proportion increasing, the shear strength and coordination number reduce and the displacements get larger. This research provides a foundation for the application of recycled ballast, and on the other hands, adding fresh ballast can be a solution to reinforce deteriorated ballast bed. [ABSTRACT FROM AUTHOR]

• Integrated management of track geometry and stiffness irregularities is proposed. • Field measurement results of track stiffness are summarized and analyzed. • Influence of track stiffness irregularity on train-track system are discussed. • Possible solutions to alleviate track stiffness irregularity are discussed. The dynamic performance of a railway track subjected to moving trains depends strongly on track support conditions. In reality, even for the well-constructed and well-maintained tracks, sleeper support stiffness and global track stiffness vary substantially along the track, which affects the train-track dynamic interactions, causing rapid track geometry degradation as well as the riding comfort and safety issues. Consequently, track stiffness irregularity (TSI, the spatial variation of track stiffness along the track) is important for railway construction and maintenance in addition to track geometry irregularities. So far, extensive research has been published on the TSI whereas the relevant issues have not been paid sufficient attention. In this paper, a summary and comments have been made in the field of TSI about the current research status and future trends from a critical point of view. Novel concepts of the critical values of TSIs and the integrated management of the track geometry and stiffness irregularities are proposed. The review presented in this work is valuable to advance the research on TSI and can help guide the design, construction and maintenance of railway tracks. [ABSTRACT FROM AUTHOR]

• A concept of wedge-shaped self-levelling railway sleeper is introduced. • The type and angle of the wedge sleeper are analysed. • The settlement correction and force behaviour of the wedge sleeper is presented. • The cooperation between 2D and 3D simulations is introduced to increase the feasibility of discrete element simulations. 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. [ABSTRACT FROM AUTHOR]