Generally refers to a string of freight cars that all carry the same commodity, frequently over long distances. Unit trains are widely used to haul such raw commodities as coal and grains, because they are less costly for railroads than mixed freight shipments. Unit trains have become a point of contention between agricultural shippers and the railroads, mainly because the shippers are increasingly being asked to fill longer unit trains (for example, 104 hoppers), which many local elevators are not equipped to handle.