car underside

A hopper car (US) or hopper wagon (UIC) is a type of railroad freight car used to transport loose bulk commodities such as coal, ore, grain, and track ballast. Two main types of hopper car exist: covered hopper cars, which are equipped with a roof, and open hopper cars, which do not have a roof.
This type of car is distinguished from a gondola car in that it has opening doors on the underside or on the sides to discharge its cargo. The development of the hopper car went along with the development of automated handling of such commodities, with automated loading and unloading facilities.
Covered hopper cars are used for bulk cargo such as grain, sugar, and fertilizer that must be protected from exposure to the weather. Open hopper cars are used for commodities such as coal, which can suffer exposure with less detrimental effect. Hopper cars have been used by railways worldwide whenever automated cargo handling has been desired. "Ore jennies" is predominantly a term for shorter open hopper cars hauling taconite by the Duluth, Missabe and Iron Range Railway on Minnesota's Iron Range.
A rotary car dumper permits the use of simpler and more compact (because sloping ends are not required) gondola cars instead of hoppers. Covered hoppers, though, are still in widespread use.
These are known as "Dual-purpose" hoppers, a type of the car referring to coal hopper car with rotary coupler on one end or on both ends. They can be used in both rotary or bottom-dump operations.

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