What is a unit train and what are the advantages and disadvantages of unit trains?

The unit train is a train that specializes in the transport of only one commodity, usually coal or grain, from origin to destination.

The unit train evolved from the rent-a-train concept for the movement of goods. Many times a shipper, who owns cars, will rent the train to another shipper for a particular period of time. For example, a typical coal unit train move would involve the transportation of 10,000 tons of coal in 100 hopper or gondola cars, each with a 100-ton capacity. The movement would be directly from the mine to an electric power-generating station with no stops in transit.. Because of the single commodity nature and the need to maintain regularly scheduled movements, empty backhauls occur. However, this drawback is offset by the high revenue-producing capabilities of the unit train resulting from the improved overall car utilization.