Comparison of the dynamic response and environmental impact between traditional and innovative railway track systems

Railways employ a range of different railway track forms. Here, a ballasted track design is compared to three different slab track solutions. The track designs are compared using both a life cycle analysis (LCA) and a methodology for simulation of three-dimensional vertical dynamic vehicle–track interaction, which has been verified versus field measurements. The interaction between vehicle and track is simulated in the time domain using an extended state-space vector approach. For the investigated load cases involving representative wheel and track irregularities, it is concluded that the maximum stress in the concrete parts is, for all designs, below the maximum flexural tensile strength. From the LCA, it is concluded that the production of steel and concrete cause most CO2 emissions, while the CO2 emissions due to maintenance activities such as tamping and rail grinding are only a small part of the overall emissions

Slab track optimisation considering dynamic train–track interaction

Slab track is a type of railway track that is frequently used e.g. in high-speed applications as an alternative to ballasted track. Slab track is also well suited on bridges and in tunnels since no ballast is required and the cross-section of tunnels can be reduced. Slab tracks generally have lower maintenance demands than ballasted track. However, if maintenance is required it may be expensive and intrusive. On the other hand, overdimensioning of slab track will lead to high environmental impact and monetary cost. This thesis aims to increase the knowledge and improve the understanding of the dynamic interaction between vehicle and track in order to allow for the optimisation of slab track.To this end, both two-dimensional (2D) and three-dimensional (3D) slab track models, and a transition zone model between slab track and ballasted track, have been developed. These models are used to simulate the vertical dynamic vehicle–track interaction in the time-domain. The computational cost of the simulation is reduced by using a complex-valued modal superposition technique for the finite element model of the track. In the 3D model, both rails are represented by beam elements, while the concrete parts are described using shell or solid elements. The simulations employ a mix of in-house and commercial codes. The influence of different irregularities, e.g. variations in track support conditions and irregularities in longitudinal level, on significant track responses such as wheel–rail contact forces, stresses in the concrete parts and pressure on the foundation is assessed. From Single-Input-Multiple-Output (SIMO) measurements carried out in a full-scale test rig, the 3D model has been calibrated and validated. The developed models have been used to improve the designs of slab track and transition zones. Based on a multi-objective optimisation problem that is solved using a genetic algorithm, the transition zone design has been optimised to minimise the dynamic loads generated due to the stiffness gradient between the two track forms. The slab track design has been optimised to minimise the environmental footprint considering the constraint that the design must pass the static design criteria described in EN\ua016432-2. This design is then employed in the dynamic model where it is shown that there is a further potential for design improvements and related CO2 savings. In particular, there may be possibilities to reduce the thickness of the concrete layers and the amount of concrete between the rails. Finally, a model of reinforced concrete has been implemented and combined with the dynamic model to assess consequences of cracking in the concrete panel and to evaluate stresses in the reinforcement bars

Simulation of vertical dynamic interaction between railway vehicle and slab track

The usage of slab track for high-speed railway lines has increased in recent decades. In Sweden, the building of new railway lines for higher speed and the selection of track design for such lines are currently being debated. Slab track has, so far, only been applied in small scale in Sweden, which implies that the knowledge and experience of such track are limited. This thesis aims to improve the understanding of the dynamic interaction between railway vehicle and slab track.The vertical dynamic vehicle–track interaction is simulated in the time domain using an extended state-space vector approach. By using a complex-valued modal superposition technique for the considered linear, time-invariant and two-dimensional track models, the computational cost of solving the associated initial-value problem is reduced. Two generic slab track models, including one or two layers of concrete slabs, are presented. The upper layer of the two-layer slab track model is described by decoupled beams of finite length, while the lower layer is a continuous beam. From the solution of the initial-value problem, wheel–rail contact forces, bending moments in the concrete panel and load distributions on the supporting foundation are evaluated. The presented models are applied to calculate the influences of track design parameters on various track responses. Furthermore, the influences of longitudinal track stiffness gradients and rail imperfections causing periodic and transient excitations are analysed. Transition zones between the one-layer slab track model and a ballasted track model are analysed. By considering a multi-objective optimisation problem solved by a genetic algorithm, the maximum dynamic loads on the track structure are minimised with respect to the selected design variables. From the solution of the optimisation problem, a non-dominated front of the objective functions is obtained illustrating potential for a significant reduction of the dynamic loads. Since the transition zones are optimised neglecting the influence of wheel and rail irregularities, a methodology is proposed to assess the robustness of the optimal design by evaluating its performance when periodic rail irregularities with different combinations of wavelength and phase, relative to the position of the transition, are applied in the model

Optimisation of slab track design considering dynamic train–track interaction and environmental impact

Modern railway tracks for high-speed traffic are often built based on a slab track design. A major disadvantage of slab track compared to conventional ballasted track is that the environmental impact of the construction is higher due to the significant amount of concrete required. In this paper, the dimensions of the rectangular cross-sections and the types of concrete used in slab tracks are optimised with the objective to minimise greenhouse gas emissions, while considering the constraint that the design must pass the static dimensioning analysis described in the European standard 16432-2. The optimised track design is also analysed using a three-dimensional (3D) model of vertical dynamic vehicle–track interaction, where the rails are modelled as Rayleigh–Timoshenko beams and the concrete parts are represented by quadratic shell elements. Wheel–rail contact forces and the time-variant stress field of the concrete parts are calculated using a complex-valued modal superposition for the finite element model of the track. For the studied traffic scenario, it is concluded that the thickness of the panel can be reduced compared to the optimised design from the standard without therisk of crack initiation due to the dynamic vehicle load. In parallel, a model of reinforced concrete is developed to predict crack widths, the bending stiffness of a cracked panel section and to assess in which situations the amount of steel reinforcement can be reduced. To reduce the environmental impact even further, there is potential for an extended geometry optimisation by excluding much of the concrete between the rails

Prediction of long-term differential track settlement in a transition zone using an iterative approach

A methodology for the simulation of long-term differential track settlement, the development of voided sleepers leading to a redistribution of rail seat loads, and the evolving irregularity in vertical track geometry at a transition between two track forms, is presented. For a prescribed traffic load, the accumulated settlement is predicted using an iterative approach. It is based on a time-domain model of vertical dynamic vehicle–track interaction to calculate the contact forces between sleepers and ballast in the short-term. These are used in an empirical model to determine the long-term settlement of the ballast/subgrade below each sleeper. Gravity loads and state-dependent track conditions are accounted for, including a prescribed variation of non-linear stiffness of the supporting foundation along the track model. In parallel, a two-dimensional (2D) non-linear finite element model of layered soil is verified versus field measurements and used to determine the support stiffness of each sleeper in the track model. The methodology is applied to a transition zone between a ballasted track and a slab track that is subjected to heavy haul traffic. Analyses of the influence of higher axle loads and the implementation of under sleeper pads on sleeper settlement are demonstrated

Simulation of vertical dynamic vehicle–track interaction using a two-dimensional slab track model

The vertical dynamic interaction between a railway vehicle anda slab track is simulated in the time domain using an extendedstate-space vector approach in combination with a complex-valuedmodal superposition technique for the linear, time-invariant andtwo-dimensional track model. Wheel–rail contact forces, bendingmoments in the concrete panel and load distributions on the supportingfoundation are evaluated. Two generic slab track modelsincluding one or two layers of concrete slabs are presented.The upper layer containing the discrete slab panels is describedby decoupled beams of finite length, while the lower layer isa continuous beam. Both the rail and concrete layers are modelledusing Rayleigh–Timoshenko beam theory. Rail receptances forthe two slab track models are compared with the receptance ofa traditional ballasted track. The described procedure is demonstratedby two application examples involving: (i) the periodicresponse due to the rail seat passing frequency as influenced bythe vehicle speed and a foundation stiffness gradient and (ii) thetransient response due to a local rail irregularity (dipped weldedjoint)

Calibration and validation of the dynamic response of two slab track models using data from a full-scale test rig

For the development of accurate and reliable simulation models, the procedure of calibration and validation against measurement data is essential. In this paper, a finite element model and a waveguide finite element model of a slab track are calibrated and validated against hammer impact measurement data from a full-scale test rig. The finite element model is three-dimensional, where the rails are modelled as Rayleigh–Timoshenko beams and the concrete slab and support layer are modelled using linear shell elements. In the waveguide finite element model, a constant track cross-section described by two-dimensional finite elements is assumed, and the vibration in the direction perpendicular to the cross-section is described by propagating waves that are decaying exponentially. Measured frequency response functions (FRFs) are compared with the corresponding calculated FRFs from the two modelling approaches. The calibration is conducted in two steps using (i) a parameter study and (ii) a genetic algorithm. For multiple excitation positions and sensor locations, both calibrated models capture the trend of the Single-Input Multiple-Output measurements with rather small deviations compared to the overall dynamic range. This implies that both models can successfully represent the dynamic response of the test rig and can be considered as validated

Multi-objective optimisation of transition zones between slab track and ballasted track using a genetic algorithm

The vertical dynamic vehicle–track interaction in a transition between ballasted track and slab track is simulated in the time domain using an extended state-space vector approach. A complex-valued modal superposition technique is applied for the linear, time-invariant and non-periodic finite element model of the railway track. By considering a multi-objective optimisation problem solved by a genetic algorithm, the maximum dynamic loads on the track structure are minimised with respect to the selected design variables. To reduce the risk of long-term degradation of track geometry due to ballast/subgrade settlement, the transition zone is designed to minimise the influence of the track stiffness gradient between the two different track forms. The methodology is demonstrated by minimising the maximum wheel–rail contact force and the maximum pressure between sleeper/panel and foundation, while the selected design variables are distributions of rail pad stiffness and sleeper spacing adjacent to the transition. From the solution of the optimisation problem, non-dominated fronts are obtained illustrating potential for a significant reduction of the dynamic loads. It is shown that the optimised design leads to a more uniform distribution of load on the foundation reducing the risk of differential track settlement. The influences of the length of the transition zone and direction of travel on the maximum dynamic loads are investigated. Prescribed irregularities in longitudinal level may be accounted for but have been neglected in the optimisation as the optimised design would be more influenced by the given irregularity than by the stiffness gradient

Dynamic interaction between vehicle and slab track – Influence of track design parameters

The vertical dynamic interaction between a high-speed railway vehicle and slab track is simulated in the time domain using a two-dimensional track model. The transient interaction problem is solved using an extended state-space vector approach in combination with a complex-valued modal superposition technique for the track model. The presented slab track model includes two layers of concrete beams, where the panels of the upper layer are described by possibly coupled beams of finite length, while the roadbed in the lower layer is a continuous beam. Both rail and concrete layers are modelled using Rayleigh-Timoshenko beam theory. The presented model is applied to calculate the influences of foundation stiffness gradient and track design parameters on various track responses. In particular, the influences of the thickness of the roadbed on the load distribution on the foundation, and of the stiffness of the rail pad on the bending moment in the concrete panels, are investigated in two demonstration examples

Simulation of vertical dynamic vehicle–track interaction using a three-dimensional slab track model

When improving track design, a better understanding of the track\u27s damage modes is needed, and the railway industry is then dependent on the availability of accurate simulations of the dynamic vehicle–track interaction. In the present study, the vertical dynamic interaction between a travelling railway vehicle and a slab track is simulated in the time domain by using an extended state-space vector approach. A three-dimensional slab track model is launched where the rails are modelled using Rayleigh–Timoshenko beam elements and the concrete panel and roadbed are represented by using either shell or solid finite elements. From the parameterised track model, which is developed in Abaqus using Python scripts, the system matrices are exported to Matlab where the simulation of the dynamic vehicle–track interaction is performed. A complex-valued modal superposition technique is employed, which reduces the computational cost of the simulation. In a post-processing step, calculated wheel–rail contact forces from the dynamic analysis are used as input to the Abaqus track model where various track responses are evaluated. In particular, the time history of principal stresses is determined at critical locations in the concrete panel. Also the influence of the speed of the vehicle on the wheel–rail contact forces, and the influence of a transverse culvert below the track (modelled as a local increase of the foundation bedding modulus) on the track stiffness variation at the rail level, are investigated. A mesh convergence study for a range of track responses has been conducted including investigations of when to use linear or quadratic elements and shell or solid elements. Finally, the presented three-dimensional models have been compared to an alternative two-dimensional model to determine in what situations a two-dimensional model is sufficient