A unit train is a type of freight train that consists of only one type of cargo, such as coal or grain, and is made up of a single set of cars. The term 'unit train' is derived from the fact that all of the cars in the train are of the same type and are usually operated by a single railroad company. The crossword solution for the term 'unit train' is CAR, which stands for 'combined automotive railway.' This refers to the fact that the cars in a unit train are typically combined into a single unit, allowing them to be moved more efficiently and quickly.