|Abstract:||Railway ballast serves as a major structural and drainage component of railroad track, but is known to degrade over time. This progressive degradation increases the amount of finer particles within the ballast over time, called fouling material, which decreases its drainage capacity and strength. Railroads commonly remove the fouling material using ballast shoulder cleaners and undercutters, however, these are expensive operations that require large machines and trained personnel. If the track is allowed to degrade for too long, significant track geometry, drainage, and stability problems can arise, creating potentially very costly derailments. Nevertheless, the point at which the fouling material should be removed from the ballast, or cleaned, is still relatively undefined. This study serves to investigate the drainage component of railway ballast and provide insight into how its drainage capacity changes with fouling so that determining a ballast cleaning threshold can be further defined. Previous studies within the railway ballast and rock fill dam permeability areas are first reviewed and compared from which it was learned that the rock fill dam studies have known of non-Darcy flow in large aggregates while the railway ballast permeability studies have not. Constant head permeability tests were then conducted using the University of Illinois Constant Head Aggregate Permeameter on clean and fouled ballast providing results that show the flow through clean ballast can be on the order of fourteen times as high as that through fouled ballast and it displays a nonlinear discharge velocity versus hydraulic gradient relationship. As the ballast becomes more fouled, however, the relationship becomes linear. Finally the Fouling Index and Void Contaminant Index are compared with the permeability results demonstrating that the Void Contaminant Index is a superior parameter to describe ballast fouling. From these findings future study recommendations are given for specifically determining when to clean ballast.