A solid ground for the understanding of track settlement in railway crossings
Railway turnouts (switches and crossings, S&C) are critical components in the railway system. They provide flexibility in traffic routes by allowing trains to switch from one track to another. To serve this purpose, the turnout consists of both movable and fixed mechanical parts, as well as systems for mechatronics and signalling. In Sweden alone, there are about 14 000 S&Cs in the 16 600 km of railway network.
Turnouts stand for significant contributions to the number of reported track faults and the total cost for railway maintenance. In 2018, the cost for maintenance of turnouts in Sweden was MSEK 530, corresponding to about 10 % of the total railway maintenance cost. Further, it was the railway component that caused most train delays. One of the main drivers 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 resulting in the degradation of rail profiles and track geometry.
One contribution to the degradation of track geometry in turnouts is differential track settlement. This is a phenomenon where the horizontal level of the supporting track substructure decreases in height over time when subjected to repeated traffic loading. Dynamic wheel–rail contact forces with high magnitudes are generated in the switch and crossing panels due to the discontinuities in rail profiles that are necessary to allow for the rerouting of traffic. Because of the turnout design and the variation in track support conditions, the load transferred into the track bed is not uniform, thus resulting in a variation in settlement along the track and irregularities in track geometry. Poor quality in track geometry induces higher dynamic wheel--rail contact forces that further increase the degradation of rail profiles and track geometry.
The present work aims to provide a methodology to improve the understanding of differential track settlement in railway turnouts. This includes predictions of the high-magnitude wheel--rail impact loads on the crossing generated by passing trains with worn wheel profiles, the distribution of contact pressure between sleepers and ballast, and the accumulated permanent deformation of the track substructure.
Another objective is to provide an accurate and generic simulation environment accounting for the multiple wheel–rail contacts in the crossing panel and considering the high-frequency dynamic interaction between the vehicle and the complete railway turnout. This simulation environment offers a safe and time-efficient complement to expensive field experiments. It also allows for an optimisation of the turnout design. Examples of design aspects considered in this thesis are selection of rail pad stiffness, implementation of under sleeper pads, and design of the bearers (sleepers). A better understanding and mitigation of wheel–rail impact loads and differential settlement in turnouts can also contribute to the reduction of other track degradation mechanisms, such as wear, plastic deformation and rolling contact fatigue of the rails.