In railroads, the bed or foundation on which the superstructure (ties, rails, etc.) rests; in common roads, the whole material laid in place and ready for travel.
In highway construction, the graded portion of a highway within top and side slopes, prepared as a foundation for the pavement structure and shoulder.
on real railroads, the foundation layer of earth on which the track is built. On a model railroad, it's a strip of wood, cork, foam, or other material that simulates the roadbed and ballast of a real railroad.