Geogrids are widely used in ballasted rail tracks for reinforcement and stabilisation. During train operation, ballast becomes contaminated or fouled due to infiltration of fines from the surface, mud pumping from the subgrade, and degradation, which decreases the performance of the geogrids. This paper presents the results of a laboratory and numerical simulation to study the effect that coal fines have on the interface between ballast and geogrid. The stress-strain behaviour of fresh and fouled ballast reinforced by geogrid was investigated via a series of large scale direct shear tests in the laboratory and numerical simulations using the Discrete Element Method (DEM). The geogrid was modelled by bonding a large number of small spheres together to form the desired geometry and apertures. Irregular particle shapes were simulated in DEM by connecting many spheres together in appropriate sizes and positions. Fouled ballast was modelled by adding a predetermined amount of miniature spheres into the voids of the fresh ballast. The DEM results were then compared qualitatively with the laboratory data, and the effects of fines on the resulting shear stress-strain of ballast and the contact forces developed in the geogrids are discussed.