• A hopper railroad car carries bulk freight such as coal.train cars image by Dmitri MIkitenko from Fotolia.com
  
  Hopper cars carry the bulk freight transported by railroads. Coal, gravel, ore, aggregate materials, chemicals and grain all travel across the rails in hoppers. Unit trains, those with one type of car heading for one destination, are often made up of hoppers on the move to fuel power plants or supply food processors. Although there are variations of modern design for hoppers, their common features allow gravity loading from the car's top, and unloading through hatches in the bottom.
  
  

Open-Top Hoppers

• The open-top hopper is often called a coal car, and for good reason: Hopper cars first came into use during the 1820s to move coal over the Delaware & Hudson Canal Co. rail lines, says an article on the website American-Rails.com. The basic hopper is a steel- or aluminum-sided car with sloped ends and bottom chutes with hatches. Aggregate material is dumped into the car from above by conveyor belt, gravity chute or crane. For unloading, the car is spotted over an unloading bin, and the chutes are opened to dump the load. Hoppers can carry up to 100 tons of aggregate and can have two to four drop-bottom chutes. A variation of the basic hopper, commonly known as a coal porter or rotary gondola, has couplers that rotate 360 degrees to allow the car to be turned over to dump its load without uncoupling from the rest of the train.
  
  

Covered Hopper

• If a bulk dry commodity needs protection from the weather during transport, it's loaded into a covered hopper. The enclosed car operates the same as the open-top hopper, although variations can be optimized to carry dry chemicals and cement, whereas others are meant to transport agricultural products including grain and sugar. Some covered hoppers look very similar to the open-top cars, but many have rounded sides to improve unloading. Unloading from the bottom can be done through gravity or pneumatic suction.
  
  

Ore Jennies

• A shorter version of the open-top hopper is the ore jennie. They are used to carry more dense, thus heavier, aggregate such as taconite pellets from iron ore. These cars are predominately used in the iron ore range of the upper Great Lakes region of Minnesota and Michigan. Unit trains of ore jennies run from the mines to ore docks on Lake Superior, where the ore is transferred to ships for transport to Canadian and American steel mills.