Mitigating long-term differential settlement in railway transition zones using auxiliary rails: A numerical study

Differential settlement in ballasted railway tracks, particularly within transition zones between different track forms, presents a significant challenge to railway infrastructure. This phenomenon, often exacerbated by a stiffness gradient arising from differences in track superstructure and substructure, typically results in a local maximum (dip) in the longitudinal track profile a few metres from the transition. Such irregularities increase dynamic traffic loading, reduce passenger comfort and increase vibrations to the surroundings. The potential of auxiliary rails to mitigate long-term differential settlement are investigated through advanced numerical modelling. Auxiliary rails are integrated into the superstructure to modify the stiffness gradient and hence the load distribution mechanisms. The developed model combines a two-dimensional vehicle model, a non-linear three-dimensional (3D) track model, and a 3D finite element (FE) representation of the substructure. For a prescribed traffic load, the model predicts the accumulated settlement using an iterative approach that captures the permanent deformation (settlement) of ballast and subgrade under repeated train loading until a specified traffic tonnage is reached. The study examines long-term differential settlement, the development of voided sleepers leading to load redistribution between rail seats, and the evolution of vertical track irregularities. The results offer valuable insights into the effectiveness of auxiliary rails as a countermeasure against long-term differential settlement in railway transition zones and provide practical guidance for the improved design and maintenance of durable, high-performance railway tracks.