What is a light engine?

Answer: B — An engine without any cars

A light engine refers to a locomotive that is operating without any attached cars or wagons. This scenario typically arises when a train needs to reposition a locomotive for servicing, to pick up or drop off cars, or when transferring equipment between locations. The term "light" suggests that the engine is not carrying any load, which distinguishes it from trains that are either hauling passengers or freight. In the context of railroad operations, understanding the function and terminology related to light engines is essential for effective communication and coordination among crew members and dispatchers. The other answers involve train configurations that include multiple cars or specific types of train operations, which do not accurately describe the function of a light engine.