The railway transition zone where the track transitions from a ballasted track to a slab track, is a crucial area that can experience amplified dynamic responses. This work aims to develop a deeper insight into the mechanisms leading to the amplified dynamic response in railway transition zones. The study employs a finite element model to investigate the amplification of total strain energy due to the phenomena of reflection and redistribution of energy close to the transition interface. The results of the study are obtained for three case studies involving non-reflecting boundary (representing an energy sink) and homogeneous material along the vertical direction of the track, and the responses are studied for individual and combined effects in comparison to a benchmark case. The findings of the study show that eliminating the phenomena results in no dynamic amplification in total strain energy in railway transition zones. The conclusion highlights the importance of understanding these phenomena in order to design an efficient railway transition structure.@en

Railway transition zones (RTZs) experience higher rates of degradation compared to open tracks, which leads to increased maintenance costs and reduced availability. Despite existing literature on railway track assessment and maintenance, effective design solutions for RTZs are still limited. Therefore, a robust design criterion is required to develop effective solutions. This paper presents a two-step approach for the formulation of a preliminary-design criterion to delay the onset of processes leading to uneven track geometry in RTZs. Firstly, a systematic analysis of each track component in a RTZ is performed by examining spatial and temporal variations in kinematic responses, stresses and energies using a finite element model of an embankment-bridge transition. Secondly, the study proposes an energy-based criterion to be assessed using a model with linear elastic material behavior and states that an amplification in the total train energy in the proximity of the transition interface is an indicator of increased (and thus non-uniform) degradation in RTZs compared to the open tracks. The correlation between the total strain energy (assessed in the model with linear material behaviour) and the permanent irreversible deformations is demonstrated using a model with non-linear elastoplastic material behavior of the ballast layer. In the end, it is claimed that minimising the magnitude of total strain energy will lead to reduced degradation and a uniform distribution of total strain energy in each trackbed layer along the longitudinal direction of the track will ensure uniformity in the track geometry.@en

Railway transition zones are the most critical part of the railway infrastructures that experience 4-8 times more degradation compared to open tracks. Despite several attempts to reduce the maintenance and operation costs in these critical zones, a robust and comprehensive solution remains unknown. In recent studies, a robust safe hull inspired energy limiting design of a transition structure was proposed for an embankment-bridge transition (without ballast layer over the bridge) to deal with operation-induced degradation. However, this solution was investigated in detail for only this particular type of transition. In this work, the scope of this mitigation measure is extended for an embankment-bridge transition with ballast running over the bridge and its performance is evaluated using a strain-energy criterion. It was concluded in the end that the safe hull inspired energy limiting design can effectively mitigate the operation-induced dynamic amplification for more than one type of railway transition zones. @en

The railway transition zone where the track transitions from a ballasted track to a slab track, is a crucial area that can experience amplified dynamic responses. This work aims to develop a deeper insight into the mechanisms leading to the amplified dynamic response in railway transition zones. The study employs a finite element model to investigate the amplification of total strain energy due to the phenomena of reflection and redistribution of energy close to the transition interface. The results of the study are obtained for three case studies involving non-reflecting boundary (representing an energy sink) and homogeneous material along the vertical direction of the track, and the responses are studied for individual and combined effects in comparison to a benchmark case. The findings of the study show that eliminating the phenomena results in no dynamic amplification in total strain energy in railway transition zones. The conclusion highlights the importance of understanding these phenomena in order to design an efficient railway transition structure.@en

Railway transition zones present a major challenge in railway track design mainly due to abrupt jumps in stiffness and differential settlements that result from crossing stiffer structures such as bridges or culverts. Despite numerous efforts to mitigate these transition effects at both the superstructure and substructure levels, a comprehensive solution remains elusive. Substructure-level interventions have demonstrated some effectiveness but are often cost-prohibitive and challenging to implement in existing operational railway transition zones. In contrast, mitigation measures at the superstructure (rail, sleepers, rail-pads, under-sleeper pads) level can be easily installed but have shown limited improvement in site measurements. This study evaluates the influence of different sleeper configurations in transition zones and reduced sleeper spacings on the operation-driven dynamic amplifications in railway transition zones, employing a recently proposed criterion based on the total strain energy in the track-bed layers (ballast, embankment, and subgrade). In addition to this, the influence of the loss of contact between sleepers and ballast (i.e., hanging sleepers), which typically results from the differential settlement, is studied. The first part of the paper provides useful insights regarding the interventions (and/or initial design) in the sleeper configuration and spacing, whereas the second part of the work highlights the need for interventions to deal with the loss of contact between sleeper and ballast. A 2-dimensional finite element model of an embankment-bridge transition was used for the analysis. The results show that it is not possible to mitigate the transition effects completely using the interventions involving sleeper spacing and configuration.@en

This short paper presents, in a summarized way, non-linear connections for a lattice model of a railway, with the intention of simulating ballast compaction at transition zones.@en

This comprehensive study addresses the persistent issue of railway transition zone degradation, evaluating the efficacy of the most commonly used mitigation measures and proposing a novel Safe Hull-Inspired Energy Limiting Design (SHIELD) of a transition structure. Firstly, this work assesses the traditional transition structures, including horizontal and inclined approach slabs and transition wedges, using commonly studied responses (kinematic response and stress) and a recently proposed criterion based on total strain energy minimization. The second part of the paper evaluates the newly introduced transition structuress (SHIELD) using the same criterion as used for the evaluation of the traditional transition structures. A detailed investigation of existing and a new design using a 2-dimensional finite element model shows SHIELD’s effectiveness in managing energy flow at transition zones and provides reasoning behind the ineffectiveness of the other commonly used transition structures. The study demonstrates the robustness and comprehensiveness of the recently developed energy-based criterion and its applicability to different types of transition zones. Moreover, it highlights the potential of SHIELD as a solution to address the complexities associated with the design of railway transition zones.@en

ransition zones in railway tracks experience strong amplification of stress and strain fields due to the passage of train over inhomogeneity. The inhomogeneity in these zones can be attributed to changes in mechanical properties of material along the longitudinal direction of the track, and to displacement/traction discontinuities at interfaces leading to an amplified response in railway transition zones (TZ) with respect to the open tracks. In this paper, different kinds of inhomogeneities are considered in isolation and in combination to study the effects on railway track components in transition zone. The first type of inhomogeneity considered is non-uniformity of materials at various levels of the track along the longitudinal direction. The second type of inhomogeneity that will be considered arises from displacement and traction discontinuities at the interface of soil and structure and at the interface of sleepers and ballast (hanging sleepers). The results provide necessary insight for the design of effective mitigation measures to prevent the amplified response in railway TZ.@en

This work addresses the contribution of the wavelength composition of the spectrum of the rail support stiffness profile to the expected long-term settlement. To that aim, purely harmonic stiffness variations of different wavelength are studied. The frequency-domain model with a double periodicity level previously developed by the first and last authors is adopted to embed the stiffness profile in one of the periodicity layers. Additional resonance velocities at which the resonance frequency of the track system coincides with the support-passing frequency or its multiples are found. The susceptibility to degradation is assessed both by quantifying the mechanical energy dissipated in the substructure under a moving train axle within one wavelength of the support stiffness variation, and the work performed by the wheel-rail contact force. It is shown that shorter wavelengths and larger standard deviations of varying ballast/subgrade stiffness result in an increasing energy dissipation in the substructure, and increase the work performed by the wheel-rail contact force, therefore leading to a reduced lifetime of the track. The energetic quantities increase for lower mean values of the stiffness profile, confirming the proneness of tracks on soft soils to degradation. The influence of varying stiffness vanishes for wavelengths of approximately 16 times the sleeper span, which is equivalent to a track length of about 10 m. High railpad stiffness values result in increased energy dissipation but the influence is limited. In general, an increasing train velocity amplifies the rate of track degradation, with no stabilizing trend in the high-speed regime (300 km/h).

@en

Locations in railway tracks where significant variations of the track properties occur are subject to increased track deterioration. To successfully mitigate this, the mechanisms leading to the increased deterioration need to be understood. To this end, this work presents a non-linear constitutive law for a lattice model able to describe the compaction behaviour of railway ballast. The parameters of the non-linear connections are tuned against lab experiments of cyclic loading tests and direct shear tests. The tuned lattice can be used with different foundation properties provided that the ballast in the track is equivalent to that of the tests. The non-linear lattice model is applied to the case of railway transitions, for which ballast compaction under train loading is studied as a cause of geometry degradation. It is observed that for the studied cases of a culvert crossing and of a ballast-slab transition, the operation-induced compaction converges monotonously to a stable situation, without leading to significant changes in the vehicle-track interaction. Ballast compaction is therefore insufficient as a stand-alone mechanism to explain a process of progressive degradation of the track geometry. Other mechanisms like autonomous differential settlement at the foundation level must be taken into account in such cases.@en

Rail grinding is widely used as a technique to both reprofile the railhead in case of wear and to remove damage. However, grinding may lead to surface burning and the formation of a white etching layer (WEL). Taking into account the rail head position, the study established an analytical thermal model based on a non-uniformly distributed heat source to predict the temperature field during grinding. The grinding temperature during a rail grinding experiment is measured through thermocouples to validate the model. In addition, the rail material response in terms of surface burn and white etching layer is analyzed in detail. Results indicate that with a grinding temperature of around 400 °C, a WEL starts to appear on the rail surface. Retained austenite is found on the ground rail surfaces, indicating the existence of martensite, resulting from the coupling effect of thermal stress and mechanical stress. A diagram is developed to describe the relationship between the grinding temperature, surface burn, and WEL.

@en

Different origination theories for squat-type rail defects are examined and confronted with experimental evidence. Based on the morphology of the three-dimensional defect crack pattern, with a leading crack directivity governed by the tangential stress history, it is shown that theories assuming dynamic wheel-rail interaction as a direct initiation mechanism of the double-lobed defect violate the causality principle. Rail surface anomalies of three categories increase the tangential stress exposure and thus the risk of defect development: pertaining to the material properties, contact geometry (including both the global scale, involving the dynamic stress level transmitted by the contact area, and the local scale, involving transient stress redistribution within this area), and contact stiffness. Both detection measures focusing on geometrical surface deviations giving rise to dynamic wheel-rail interaction (such as axle-box acceleration measurements) and the idea of a critical diameter for such imperfections are inadequate. Instead, and apart from the surface geometry, steel micro-cleanliness and chemical composition, phase transformation mechanisms of surface material (due to operational conditions or treatment by grinding or milling) and welding deserve attention. Fluid entrapment remains a potentially contributing factor in early growth, while crack face oxidation and corresponding volumetric expansion may contribute at any stage after initiation.

@en

A periodically supported beam on a visco-elastic half-space is considered to model the vibration of railway tracks. The viscosity of the half-space is assumed to be of the Kelvin-Voigt type. Making use of the concept of equivalent dynamic stiffness, the reaction of the half-space to the sleepers is replaced by a system of identical spring located under each sleeper. The frequency-dependent equivalent stiffness of the springs is a function of the phase shift of vibrations of neighbouring supports. The equivalent stiffness is derived analytically employing the contour integration technique, resulting in a comprehensive expression for different phase velocities of the waves in the beam with respect to the wave speeds of the half-space. Apart from the Rayleigh type surface wave (quasi-elastic wave), an extra visco-elastic surface wave may exist in a visco-elastic half-space depending on the parameters of the half-space and the frequency range. The existence of this second surface wave has not been addressed within the field of train-induced ground vibration. The importance of this wave for the equivalent stiffness is analysed. An effective method to determine the frequency range for the visco-elastic surface wave to exist is proposed.@en

The effect of large-scale variation in the support stiffness on railway track degradation is studied using a frequency-domain approach. The model used can deal with parametric excitation due to both the discrete sleeper spacing and arbitrary large-scale spatial track non-uniformity. Adopted stiffness profiles are based on realistic datasets in the literature. The sensitivity to degradation is assessed by quantifying the energy dissipation in the substructure over the influence zone. Results show that the effect of spatial stiffness variation generally increases with the speed, for any subgrade condition; system resonance however leads to increased degradation at resonance speeds and increases with the mean value of the track stiffness. The speed is shown to have a larger influence in the presence of non-uniformity than it has for uniform track with a mean value of the same non-uniform track stiffness, independent of this mean value. In general, support stiffness non-uniformity and poor track conditions (in terms of a low overall stiffness) may have comparable effects; the combination of both is a worst-case scenario. Predictions are independent from the randomness for measured datasets and have therefore general validity. Further, an excellent correlation is found between the spatial variation of the dynamic track stiffness, the differential energy dissipation in the substructure, and the work performed by the moving contact load with respect to the track, independent of the train speed. This confirms existing empirical evidence of the dynamic track stiffness for non-uniform track as an indicator for degradation.@en

A novel high-strength steel design is proposed, with a fine bainitic microstructure free from inter-lath carbides, for railway crossings applications. The design is based on the phase transformation theory and avoids microstructural constituents like martensite, cementite and large blocky retained austenite islands in the microstructure which are considered to be responsible for strain partitioning and damage initiation. The designed steel consists of fine bainitic ferrite, thin film austenite and a minor fraction of blocky austenite which contribute to its high strength, appreciable toughness and damage resistance. Atom probe tomography and dilatometry results are used to study the deviation of carbon partitioning in retained austenite and bainitic ferrite fractions from the T0/T0ʹ predictions. A high carbon concentration of 7.9 at.% (1.8 wt%) was measured in thin film austenite, which governs its mechanical stability. Various strengthening mechanisms such as effect of grain size, nano-sized cementite precipitation and Cottrell atmosphere at dislocations within bainitic ferrite are discussed. Mechanical properties of the designed steel are found to be superior to those of conventional steels used in railway crossings. The designed steel also offers controlled crack growth under the impact fatigue, which is the main cause of failure in crossings. In-situ testing using micro digital image correlation is carried out to study the micromechanical response of the designed microstructure. The results show uniform strain distribution with low standard deviation of 1.5% from the mean local strain value of 7.7% at 8% global strain.@en

This study addresses the contribution of spatial variance in the railway track support stiffness to the expected long-term track degradation. Hereto, a novel frequency-domain model is developed with a double periodicity ‘layer’, capable of dealing with both sleeper periodicity and arbitrary non-uniformity in track properties. The model application focuses on a locally reduced support stiffness (hanging sleeper) along the track. The resulting susceptibility to degradation is assessed by quantifying the mechanical energy dissipated over the influence length under a moving train axle. Different descriptions of this energy amount are benchmarked with respect to their predictive value. In the presence of a degraded sleeper support, hanging sleepers are found to develop faster with increasing train speed; the speed effect may be estimated as roughly linear. Moreover, degradation increases progressively with an increasing local relative stiffness reduction. Coincidence of the train speed corresponding to the sleeper passing frequency with the first resonance peak of the system leads to severely increased degradation; increased damping however attenuates dissipation peaks at resonant speeds and shifts their position upwards. The effect of a degraded support is most significant on soft subgrades. The effect of multiple degraded sleeper supports increases up to three sleepers, for any train speed. With respect to the system parameters, particularly the railpad stiffness has significant effect; especially for high-speed tracks a high pad stiffness is very unfavorable. Other effective control parameters in the case of degraded sleeper supports are the sleeper spacing and the rail cross-sectional properties; for example replacing a 54E1 with a 60E1 rail profile may reduce energy dissipation with roughly 30% on high-speed track. An increasing unsprung vehicle mass is unfavorable for track degradation, again with the effect increasing with the train speed. The developed methodology is shown to have significant potential with respect to railway track design in terms of multi-parametric optimization for concrete cases with a given input in terms of soil properties and operational regime.@en

This paper studies the effect of railway track design parameters on the expected long-term degradation of track geometry. The study assumes a geometrically perfect and straight track along with spatial invariability, except for the presence of discrete sleepers. A frequency-domain two-layer model is used of a discretely supported rail coupled with a moving unsprung mass. The susceptibility of the track to degradation is objectively quantified by calculating the mechanical energy dissipated in the substructure under a moving train axle for variations of different track parameters. Results show that, apart from the operational train speed, the ballast/substructure stiffness is the most significant parameter influencing energy dissipation. Generally, the degradation increases with the train speed and with softer substructures. However, stiff subgrades appear more sensitive to particular train velocities, in a regime which is mostly relevant for conventional trains (100–200 km/h) and less for high-speed operation, where a stiff subgrade is always favorable and can reduce the sensitivity to degradation substantially, with roughly a factor up to 7. Also railpad stiffness, sleeper distance and rail cross-sectional properties are found to have considerable effect, with higher expected degradation rates for increasing railpad stiffness, increasing sleeper distance and decreasing rail profile bending stiffness. Unsprung vehicle mass and sleeper mass have no significant influence, however, only against the background of the assumption of an idealized (invariant and straight) track. Apart from dissipated mechanical energy, the suitability of the dynamic track stiffness is explored as an engineering parameter to assess the sensitivity to degradation. It is found that this quantity is inappropriate to assess the design of an idealized track.@en

This study examines the properties of stratified surface layers on rails in service and presents a hypothesis explaining their origin. The stratified layer consists of a white etching top layer and a brown sublayer. The metallurgical composition and properties of this sublayer are found to match with that of globular bainite. The occurrence of stratification in the surface layer is explained by the thermomechanical cycle for a material point on the rail surface under wheel-rail contact. Difference in the surface and subsurface cooling rates after reaching the austenitisation temperature may lead, depending on the chemical steel composition, to the generation of two different phases (martensite and bainite) and stratification. The exclusive occurrence of sandwich layers on rails that have been in service is attributed to the hardening of the top layer, leading to a reduced thermal conductivity, which gains relevance at an increasing depth. The granular morphology of the bainitic sublayer, exhibiting weak globular inclusions, facilitates the initiation and the propagation of transverse cracks, thus contributing to the development of RCF.

@en

The extensive usage of railway infrastructure demands a high level of robustness, which can be achieved partly by considering (and managing) the track and rolling stock as one integral system with due attention to their interface. A growing number of infra managers consider, in this framework, the track-friendliness of vehicles that have access to their tracks as a key control parameter. The aim of this study is to provide further insight into potential contributions to track-friendliness, assessed in relation to track deterioration mechanisms and cost, understanding how potential benefits are best to be utilised. Six proposed freight bogie design measures are evaluated with respect to the improvement in curving behaviour, switch negotiation and related track degradation mechanisms. To this purpose a sensitivity analysis has been carried out by means of track–train simulations in the VAMPIRE® multi body simulation software. Additionally, the impact on track deterioration costs has been calculated for those track-friendly design modifications identified as most promising. Conclusions show that the standard Y25L freight bogie design displays rather a track-friendly behaviour. Tuning the primary yaw stiffness shows a high potential to further improve track-friendliness, significantly reducing track deterioration cost at narrow radius curves and switches (by, respectively, 30% and 60%). When calculating the overall deterioration cost for the travelled route, the calculation model should include a well-balanced representation of switches and narrow radius curves.

@en