In this paper, a finite-element (FE) model including the track super- and substructure components was developed for seismic analysis of railway track. In the developed model, the beam elements were used to define rails and sleepers as components of track superstructures. Moreover, a series of lumped masses with springs and dashpots was considered for modeling the ballast, subballast, and subgrade as track substructure components. In order to validate the FE model, the results of seismic numerical analysis were compared with the outputs of a shaking table test. In the next stage, using the validated FE model, the efficient excited length of the track during the Kobe earthquake was determined around 40 m as the length that created the maximum internal forces and deformations in track components. Finally, using the evaluated effective length and adopting the artificial accelerograms with various peak ground acceleration (PGA) values in the range of 2 to 10 $\mathrm{m}/{\mathrm{s}}^2$ (200 to 1,000 Gal) as input motion, the seismic behavior of the ballasted railway track was studied, and consequently the effect of PGA values on rail buckling length and its deformations as well as rail and sleeper internal forces were investigated.

Short-span bridges are one of the most frequent infrastructures along railway tracks where railway track stiffness suddenly changes. The sudden variation in the vertical stiffness of railway tracks increases dynamic loads and causes numerous defects in ballasted railroads. Therefore, improving the dynamic performance of railway tracks can be conducted by constructing countermeasures along the transition zone. In this regard, the approach slab is a practical technique used in railway-bridge transition zones. As the dimensional shape of the approach slab plays a significant role in the dynamic response of transition zones, this study evaluated the effects of its main geometric parameters. A three-dimensional model of the railway portal bridge, including the approach slab, was built using the finite element method and analyzed by imposing moving wheel loads as a series of acting force points along rail elements. The model was validated with field-obtained results acquired through a laser/camera-based measuring technique. Then, some sensitivity analyses were performed to find optimized geometric dimensions of approach slabs to improve the dynamic behavior of railway-bridge transition zones. Obtained results of the geometrical sensitivity analysis show that when the geometrically optimized approach slab is used along the railway-bridge transition zone, the displacements of rail and ballast are decreased by 24% and 18%, respectively.

Abrupt stiffness variations along the railway track may increase the geometrical and mechanical defects of railway lines. The conjunction points of a railway track with concrete and ballast pavements, which are called slab-ballasted track transitions, are one of the main areas where vertical track stiffness changes sharply. Therefore, the potential benefits of a combined transition system along the slab-ballasted transition, made of an approach slab and additional rails, are studied in this paper. For this purpose, a vehicle-track-substructure interaction model, which included three main segments of the railway track (slab track, transition zone, and ballasted track) was programmed based on the finite element method. A test line with the mentioned combined transition system was built to measure the railway track responses through field study. Then, the three-dimensional (3D) numerical model was validated using the results obtained from the experimental tests. Afterwards, a number of parametric studies were performed to analyze the dynamic responses of the combined transition zone. The results indicated that this type of transition system promotes a smoother stiffness transition between the slab track segment and the ballasted track segment by making the transition in three gradual steps. The track displacements in the analyzed case-study gradually increased by about 22%, 28%, and 34% along the combined transition zone in the junction points of the slab and ballasted tracks.