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A hopper car () or hopper wagon () is a type of railroad freight car that has opening doors or gates on the underside or on the sides to discharge its cargo. They are used to transport loose solid Bulk cargo such as coal, ore, cereal, and track ballast. Plastic pellets and some finely ground material, similar to flour, are transported in hopper cars that have pneumatic unloading. The bottom gates on the pneumatic hoppers connect to a hose attached to industrial facilities' storage tanks. Air is injected to fluidize the railcar contents for unloading. The hopper car was developed in parallel with the development of automated handling of such commodities, including automated loading and unloading facilities.
Hopper cars are distinguished from gondola cars, which do not have opening doors on their underside or sides. Gondola cars are simpler and more compact because sloping ends are not required, but a rotary car dumper is required to unload them. Some "dual-purpose" hoppers have a rotary coupler on one or both ends, so they can be used in both rotary and bottom-dump operations.
Removable canvas covers are sometimes used to protect moisture sensitive commodities in open hopper cars. Closed hopper cars have a metal top with waterproof loading hatches, which provides superior protection. These loading hatches along the top of the covered hopper may be a single long opening along the centerline or a pattern of multiple round or square openings positioned to allow uniform weight distribution when loading the car.
Some covered hoppers have two to four separate bays, with chutes at the bottom to direct unloading contents.
Historically, open hopper cars were used to carry coarse mined products like coal, ore, and gravel, while Boxcar were used for granular materials requiring protection from the elements.
Weatherproof covers were added to hopper cars, creating the covered hopper. Early production emphasized two-bay cars very similar to open coal hoppers and suitable for materials of similar density, like Portland cement or rock-salt. Some cars were available in the 1910s, and became more common by the 1940s. These early cars were volume-limited for less dense commodities like grain or sugar, so later designs include longer covered hopper cars with higher sides and three or more bottom bays. Increasing axle load limits have allowed some of the heavier loads formerly assigned to two-bay hoppers to be assigned to larger, more efficient three-bay hoppers.
Some covered hopper cars retain the conventional centersill as a strength member transmitting compression and tension Force from one car to the next. Beginning in the 1960s, designs distributing these forces along the sides of the car eliminated the centersill beam to simplify bulk material handling with wider hopper openings reducing the tendency for bridging to restrict gravity flow when unloading the car.
Increase in wheel loads has important implications for the rail infrastructure needed to accommodate future grain hopper car shipments. The weight of the car is transmitted to the rails and the underlying track structure through these wheel loads. As wheel loads increase, track maintenance expenses increase and the ability of a given rail weight, ballast depth, and tie configuration to handle prolonged rail traffic decreases. Moreover, the ability of a given bridge to handle prolonged rail traffic also decreases as wheel loads increase. The axle load is twice the wheel load.
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