What defines an engine as used in train or yard service?

An engine, in the context of train or yard service, is defined as a unit propelled by a single control. This is because the primary function of an engine is to provide the necessary power to move the train or a series of cars, ensuring that all operations can be managed efficiently from one control point. Engines are designed to manage the dynamics of acceleration, braking, and speed control all from a single operator's position. This encompasses everything from conventional locomotives to versatile switch engines, which are critical in yard operations for moving freight cars around.

In contrast, a combination of passenger cars or a group of freight cars refers to rolling stock that relies on an engine for movement but does not constitute an engine itself. The distinction lies in the engine being the active component that generates the power for transport, while cars serve as passive transport units that are moved by the engine. Additionally, a unit operated by multiple controls might suggest a more complex operation setup but does not fit the straightforward definition of an engine designed for singular control functionality in standard train operations.