Abstract: Performance of the ballast layer largely depending on its gradation. However, ballast gradation would change over time due to fines accumulation in the void among the large aggregate particles, which is often termed as ballast fouling. Many researches have focused on studying the fouled ballast behavior by mixing new ballast aggregates and fines, such as coal dust, clay, sand, etc. However, behavior of the fouled ballast due to degradation has not been well understood. Furthermore, the fundamental mechanism that governs transport and accumulation of fine particles in the ballast layer is not thoroughly investigated. The transport paths and the sediment pattern of the fouling materials in the ballast layer remains unclear. This presentation presents the findings of degraded ballast behavior and recent attempts of developing a high-fidelity coupled fluid-particle computational fluid dynamics (CFD) and discrete element method (DEM) simulation approach to investigate fine particles transport and accumulation in the ballast layer.