In the event of a signal that indicates “Stop,” what should the locomotive engineer do?

When a signal indicates "Stop," the locomotive engineer must instantly bring the train to a stop and assess the situation because safety is the highest priority in rail operations. Stopping immediately allows the engineer to prevent potential accidents, collisions, or other hazardous situations. After stopping, the engineer can then evaluate the environment and contact the appropriate personnel for further instructions. This immediate and decisive action is a critical part of adhering to operational protocols and ensuring the safety of the train, its crew, and passengers.

The other choices fail to emphasize the importance of stopping promptly. Continuing at the same speed while merely observing creates a risk of not being able to react in time to any dangers. Activating warning lights and reducing speed might signal to others but does not ensure that the situation is managed effectively since the train could still be moving toward a hazard. Notifying the control center and proceeding with caution implies moving without fully stopping, which undermines the intent of the "Stop" signal and could lead to unsafe conditions.