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Double-stack rail transport
Double-stack rail transport is a form of intermodal freight transport where intermodal containers are stacked two high on railroad cars. Introduced in North America in 1984, double stack has become increasingly common there, 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 costs per container. On most North American railroads, special well cars are used for double-stack to reduce the needed vertical clearance and to lower the center of gravity of a loaded car. In addition, the well car design significantly 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 on shipments within North America.
Double-stack rail operations are growing in other parts of the world, but are often constrained by clearance and other infrastructure limitations.
==Weights==
Containers have weight limits designed to fit road trucks, which have clearly smaller weight limits than trains. A common limit for railways is per meter train length and per axle. A four axle container car can take . Since a container is limited to (plus a rail car), single stacking clearly does not use the load capacity of the railway. A container is limited to and two such can fit into a car for a container, or even three if double-stacking, but not four unless very high axle load is permitted. The North American railways permit two or four containers as shown in the images on this page.
Another consideration is the maximum weight of a train. A maximum length train in Europe, long can have 50 container cars with a total weight of , and more if 20 ft containers are included. This is not so far from the limit using standard European couplers. Double-stacking requires allowing higher train weight to be meaningful, since it is higher train weights that saves costs. In the U.S., the AAR coupler used allows much higher train weight.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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