TRACK BED

A 'track bed' is the term used to describe the groundwork onto which a railway track is laid. Trackbeds of disused railways are sometimes used for recreational paths or new light rail links.
According to Network Rail, the "trackbed" is the layers of ballast and sub-ballast above a prepared sub-grade/formation. It is designed primarily to reduce the stress on the sub-grade.
The trackbed can significantly influence the performance of the track especially ride quality of passenger services.