Covered hoppers are the most common type of railcar and represent almost one third of the national fleet.
The cars are loaded through hatches on the roof of the car and are unloaded through gates at the bottom of the car. Hatches can be round or "trough" in design. The interiors of the cars are divided into sections or "hopper bays."
Covered hoppers are available in a variety of sizes designated by the cubic capacity (the cubic feet of commodity that can be loaded). Typically, the denser the commodity being loaded the smaller the railcar needed as the weight of the product cannot exceed the load limit stenciled on the side of the car. The standard covered hopper carries between 4,600 and 4,750 cubic feet of product.
As the name implies, a small cube covered hopper is simply a covered hopper that has a lower cubic capacity than a "normal" covered hopper. They typically carry between 2,700 and 3,500 cubic feet of product. Small cars like these, which usually have 2-3 hopper bays, are used to carry heavier commodities such as sand and gravel.
Again, as the name implies, jumbo covered hoppers are a larger covered hopper which can carry over 5,000 cubic feet of product. "Jumbos" are used to carry less dense commodities such as distillers dried grain (a bi-product of ethanol production), corn cobs, or plastic pellets to be used in manufacturing.