Railway Accidents

Track bed and foundation
Railway tracks are generally laid on a bed of stone track ballast or track bed, in turn is supported by prepared earthworks known as the track formation. The formation comprises the subgrade and a layer of sand or stone dust (often sandwiched in impervious plastic), known as the blanket, which restricts the upward migration of wet clay or silt. The track and ballast form the permanent way. The term foundation may be used to refer to the ballast and formation, i.e. all man-made structures below the tracks.
Additional measures are required where the track is laid over permafrost, such as on the railway to Tibet, such as transverse pipes through the subgrade to prevent that subgrade from melting. These pipes allow cold air to penetrate the formation.
The sub-grade layers are slightly sloped to one side to help drainage of water.