Double-stack rail transport is a form of intermodal freight transport in which railroad cars carry two layers of intermodal containers. Invented in the United States in 1984, it is now being used for nearly 70% of United States intermodal shipments. Using double stack technology, a freight train of a given length can carry roughly twice as many containers, sharply reducing transport costs per container. On United States railroads, special well cars are used for double-stack shipment to reduce the needed vertical clearance and to lower the center of gravity of a loaded car. In addition, the well car design reduces damage in transit and provides greater cargo security by cradling the lower containers so their doors cannot be opened. A succession of larger container sizes have been introduced to further increase shipping productivity in the United States.
Double-stack rail operations are growing in other parts of the world, but are often constrained by clearance and other infrastructure limitations.
