As railway networks experience increasing operational demands, assessing stress distribution in critical components is vital for structural integrity and safety. This study focuses on the longitudinal stress behaviour in the Improved Switch Expansion Joint (ISEJ), a key element designed to accommodate rail thermal expansion and contraction. Using finite element method (FEM) simulations validated with experimental data from live track operations, the analysis explores the influence of varying friction coefficients between the Stock Rail, Gap Avoiding Rail (GA Rail), Tongue Rail, and CI Chair on stress distribution. A detailed parametric study is conducted across four sections of the ISEJ: Sects. 1 and 2 on the Fixed Rail Side (where the Stock rail is bolted to the GA Rail), and Sects. 3 and 4 on the Movable Rail Side (where the Tongue rail contacts the GA Rail). Sections 1 and 4 correspond to regions of minimum cross-sectional area, while Sects. 2 and 3 are positioned between sleepers. The results demonstrate that increasing the friction coefficient leads to a notable reduction in longitudinal stress. Specifically, maximum stress reductions of 10.73%, 8.37%, 18.27%, and 10.55% are observed at Sects. 1 through 4, respectively. These findings underline the critical role of interface friction in stress behaviour and suggest that optimising frictional conditions can significantly improve the load-carrying capacity and longevity of the ISEJ. This work provides actionable insights for railway engineers, contributing to safer and more durable track system designs under growing service loads.