track ballast

Track ballast forms the trackbed upon which railroad ties (US) or railway sleepers (UK) 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. The term "ballast" comes from a marine shipping term for the stones used to weigh down a ship.
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