Unit Train

A unit train, also called a block train is train in which all the cars making it up are shipped from the same origin to the same destination. This saves the hassle of assembling and disassembling trains at rail yards near the origin and destination, however, unit trains are only economical for high-volume customers. Since unit trains often carry only one commodity, cars are of all the same type, and sometimes the cars are all identical.
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A train of a specified number of railcars, perhaps 100, which remain as a unit for a designated destination or until a change in routing is made. (MARAD2)

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.