This paper presents the results of laboratory investigations into the deformation of coal fouled ballast stabilized with geogrid at
various degrees of fouling. A novel track process simulation apparatus was used to simulate realistic rail track conditions subjected to cyclic loading, and the void contamination index (VCI) was used to evaluate the level of ballast fouling. The experimental results show that coal fines act as a lubricant, causing grains of ballast to displace and rotate, and as a result, accelerate its deformation. However, coal fines also reduce ballast breakage because of a cushioning effect, that is, by reducing interparticle attrition. The inclusion of geogrid at the interface between the layer of ballast and subballast provides additional internal confinement and particle interlocking via geogrid apertures, which reduces deformation. A threshold value of VCI540% is proposed to assist practitioners for conducting track maintenance. If the level of fouling exceeds this threshold, the geogrid reinforcement significantly decreases its effectiveness and the fouled ballast exhibits a premature dilation. Based on the experimental results, an equation incorporating the VCI was proposed to predict the deformation of fresh and fouled ballast. This equation improves track design and assists in favorable decision support for track maintenance. Additionally, the discrete element method (DEM) was also used to capture the deformation of fouled ballast subjected to cyclic loading, whereas the DEM results were compared with experimental observations.