1) Foundation for a train track 2) Foundation with ties 3) Highway foundation 4) Street surface 5) Track foundation 6) Train track foundation 7) Train track supporter 8) Under the crossties 
• (n.) 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.
(from the article `railroad`) ...equipment make it possible to dig deeper cuts through hillsides and to make higher fills where necessary to smooth out the profile of the track. ...
Road'bed` noun 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. 
[n] - a bed supporting a road
noun a bed supporting a road