How should the engineer respond if the train's speed is indicated to be over the limit?

When the train's speed is indicated to be over the limit, the appropriate response is to reduce speed by applying brakes. This action is crucial for maintaining safety and adherence to operational protocols. If the train exceeds its speed limit, it could pose a risk to passenger safety, equipment integrity, and overall operational stability.

Applying the brakes promptly allows the engineer to control the train's speed effectively and bring it back within the acceptable range. This practice demonstrates a commitment to safe train operations and reflects adherence to regulatory and procedural standards designed to prevent accidents and ensure the safety of all individuals on board.

The other choices do not align with safe operational practices. Ignoring the warning can lead to dangerous situations. Contacting the control center may be necessary in certain circumstances, but the immediate priority should always be to ensure the train slows down to a safe speed. Waiting for further instructions from passengers is not appropriate, as passenger input on train operations does not constitute a method of ensuring safety.