General knowledge — feature a train must have A train always has _____. Choose the thing necessarily present for a train as a locomotive set.

Reasoning
In conventional rail transport, a train requires a motive power unit (an engine/locomotive or powered driving car) to move.

Option check

• Rails — typical for railways, but the item asked is part of the train itself.
• Driver — some systems can be automated.
• Guard — not universal.
• Engine — provides tractive effort; necessary component of the train consist.

Final Answer
Engine.