ABSTRACT
The track substructure includes the ballast, subballast, and subgrade layers that support the track superstructure of rails and ties. Track substructure behavior has a significant influence on track superstructure stability and performance as well as vehicle dynamics. The main function of the track substructure is to support the applied train loads uniformly and without permanent deformation that might affect the track geometry. This chapter focuses on the properties and roles of track substructure components of ballasted railway track structure, although other track structure types are briefly discussed.
