Train unit

CAR

Think of what a single passenger or freight compartment is called when it's part of a longer railway vehicle. It’s not the engine or caboose.

The clue "Train unit" refers to a single vehicle in a train, which is commonly called a "car." In this context, "car" doesn’t mean an automobile—it’s the railroad term for a passenger or freight compartment linked together to form a train. The answer plays on the double meaning of "car," but the clue makes it clear we’re talking about trains, not cars on the road.