This paper presents the results of an experimental study of coal-fouled ballast reinforced with geogrid, at various degrees of fouling and subjected to cyclic loading. A novel Track Process Simulation Testing Apparatus was used to realistically simulate fouled rail track conditions. The laboratory results demonstrated that coal fines acted as a lubricant, causing ballast aggregates to displace and rotate and, as a result, increase the deformation of ballast. However, coal fines also reduced ballast breakage somewhat because they fill the voids between the ballast particles and coat surface of ballast aggregates which reduce the inter-particle attrition. The placement of a geogrid at the interface between the ballast and sub-ballast layers provides extra internal confinement and interlocks the grains of ballast in its apertures, which also reduces ballast deformation. Based on laboratory results, a threshold value of VCI=40% is proposed to assist practitioners in conducting track maintenance as fouling beyond this threshold significantly reduces the reinforcement effect of geogrid so that fouled ballast experiences premature dilation leading to track instability. A novel equation incorporating the Void Contaminant Index and number of load cycles is also introduced to predict the deformation of fouled ballast, improve the design of rail tracks and help make the correct decisions with regards to track maintenance.