Track ballast forms the trackbed upon which railway sleepers (UK) or railroad tie

Railroad tie

A railroad tie/railway tie , or railway sleeper is a rectangular item used to support the rails in railroad tracks...

s (US) are laid. It is packed between, below, and around the ties. It is used to facilitate drainage

Drainage

Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...

 of water, to distribute the load from the railroad ties, and also to keep down vegetation

Vegetation

Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...

 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

Crushed stone

Crushed stone or angular rock is a form of construction aggregate, typically produced by mining a suitable rock deposit and breaking the removed rock down to the desired size using crushers...

, although ballast has sometimes consisted of other, less suitable materials.