Ballast is most commonly used as a structural component of rail track due to its high load bearing capacity, resiliency and rapid drainage. However, ballast along coal freight corridors often becomes fouled due to spilling of coal from moving wagons, in addition to further track deterioration attributed to particle breakage and pumping of soft subgrade. Highly fouled ballast needs to be cleaned or replaced to maintain the desired track resiliency, load bearing capacity and the track alignment. In order to identify the risk associated with fouling, it is important to accurately assess the amount of fouling. In this paper, the use of Void Contaminant Index (VCI) compared to other mass-based indices is critically examined. A series of isotropically consolidated drained triaxial tests using a large scale cylindrical triaxial apparatus have been conducted on both clean and fouled ballast with varying VCI to assess the stress–strain and degradation characteristics. In accordance, a non-linear shear strength envelope for clean and fouled ballast is presented in a non-dimensional form. The maximum permissible train speed under different level of fouling is also proposed.