unit train, freight train composed of cars carrying a single type of commodity that are all bound for the same destination. By hauling only one kind of freight for one destination, a unit train does not need to switch cars at various intermediate junctions and so can make nonstop runs between two terminals. This reduces not only the shipping time but also the cost. The unit train was introduced by American railroad companies in the 1950s so that they could offer lower shipping rates and thereby make their freight service more marketable. Initially, unit trains were used primarily to haul coal from mines to power plants. By the late 20th century about 50 percent of the coal shipped in the United States was carried by these trains. Other forms of bulk cargo, such as grain and cement, were also transported in this fashion.

To fully exploit the advantages of the unit train and to extend this service to shippers of manufactured goods, American railroads in the second half of the 20th century redesigned their equipment. They developed larger freight cars, many of which are specially constructed to carry particular commodities. The 10,000-cubic-foot (280-cubic-metre) boxcar, for example, is three times larger than the standard car and can economically transport such items as automobile parts and television sets. Another key innovation is the tri-level rack car capable of carrying 12 to 15 finished automobiles from assembly points to distribution points. Though most widely employed in the United States, unit trains equipped with these and other types of high-volume freight cars are also used in Canada and various European countries on a limited scale.