Wheel–Rail Impact Loads and Track Settlement in Railway Crossings

Abstract: Turnouts (Switches & Crossings, S&C) are critical components of a railway track requiring regular maintenance and generating high life cycle costs. A main driver for the high maintenance costs is the need to repair and replace switch rails and crossings as these components are subjected to a severe load environment. Dynamic wheel--rail contact forces with high magnitudes are often generated in the switch and crossing panels due to the discontinuities in rail profiles, resulting in a degradation of track geometry. One critical contribution to the track geometry degradation is track settlement. It is a phenomenon where the horizontal level of the ballasted track substructure decreases in height over time when subjected to traffic loading. Due to the design of the turnout and the variation in track support conditions, the load transferred into the track bed is not uniform and the resulting variation in settlement leads to irregularities in track geometry. Poor quality in track geometry induces higher dynamic wheel--rail contact forces and increases the degradation rate resulting in further differential track settlement, and possibly increased wear, plastic deformation and rolling contact fatigue of the rails. Thus, it is important to understand how settlement evolves under repeated loading to support product development and maintenance procedures of S&C, to provide a more uniform load distribution on the ballast and a more stable track geometry. The current work aims to provide a methodology to increase the understanding of track settlement in railway turnouts. Different numerical models are used to simulate the dynamic vehicle--track interaction and predict the wheel--rail impact loads in the crossing panel.The calculated contact pressure between sleepers and ballast is used as input for calculating the track settlement. Both empirical and constitutive settlement models are applied to predict settlement for a large number of load cycles (wheel passages). The material behaviour of the track substructure under repeated loading is investigated using a three-dimensional finite element model. A parameter study is performed to determine the influence of train and track parameters on the impact load generated at the crossing. The investigated train parameters include vehicle speed, lateral wheelset position and wheel profile, while the track parameters are rail pad stiffness, sleeper base area and implementation of under sleeper pads (USP). The study shows that the magnitude of the impact load is influenced more by the wheel--rail contact geometry than by the rail pad stiffness. Among the investigated parameter combinations, the most effective mitigation measures to reduce sleeper--ballast contact pressure are the implementation of USP and increasing the sleeper base area.