Railways offer an efficient and economic transport mode in many parts of the developed countries including Australia, China and USA. Ballast layer is designed as a load bearing layer for rail tracks and to be free draining, but when the ballast voids are wholly or partially impeded due to the intrusion of fine particles, the ballast can be considered to be fouled. Fouling causes a reduction in the drainage capacity of ballast, thereby reducing the track resiliency and triggering high maintenance costs. Geosynthetics are commonly used in railway construction for reinforcement and stabilization purposes. When railway ballast becomes fouled the beneficial effect of the geosynthetics could decrease significantly. This paper presents a study of how the interface behavior of geosynthetics-ballast copes with fouling using discrete element modelling (DEM) of large-scale direct shear tests. A series of large-scale direct shear tests for coal fouled ballast were carried out in the laboratory and were then simulated in the DEM. Shear stress-strain and volumetric dilation responses obtained from the DEM simulations were in reasonable agreement with those measured experimentally. The contact force distributions of fresh and fouled ballast were captured and shown that the fouled ballast exhibited higher number of contact forces compared to the fresh ballast assembly. This is due to coal fines accumulated in voids among large particles then partially carry and transmit contact forces across the assembly. Strains developed horizontally across the geogrid were also analyzed in this study.