Good quality track ballast is made of crushed stone. The sharp edges help the particles interlock with each other.
Track ballast forms the trackbed upon which railway sleepers (UK) or railroad ties (US) are laid. It is packed between, below, and around the ties. It is used to facilitate drainage of water, to distribute the load from the railroad ties, and also to keep down vegetation that might interfere with the track structure. This also serves to hold the track in place as the trains roll by. It is typically made of crushed stone, although ballast has sometimes consisted of other, less suitable materials.