The performance of ballasted railway tracks is largely controlled by the condition of the ballast bed. As shown on figure 1, a typical active railroad cross-section is composed of a clean ballast layer, then a transition layer of mostly clean ballast and finally a fouled ballast (or subballast) layer directly in contact with the subgrade. Subballast is sand or gravel used to provide drainage and, along with ballast, to distribute the dynamic loading applied by trains into the subgrade. The ballast and subballast systems are commonly referred as the granular layer supporting track with a design thickness of typically 4575 cm.
