When does the train engineer need to take action upon a mode change signal?

The correct response is that the train engineer needs to take action within 15 seconds of a mode change signal being activated. This timeframe is critical as it provides the engineer with a clear guideline to respond appropriately to changing operational conditions.

Taking action promptly upon receiving a mode change signal is essential for maintaining safety and adhering to operational protocols. A delay beyond this specified duration could lead to potential safety hazards or disruptions in train operations, as the engineer is required to adjust the train's operation according to the new mode indicated by the signal. The 15-second requirement allows enough time for the engineer to evaluate the situation and implement necessary changes without compromising safety or efficiency.

The other options may suggest varying degrees of urgency, but only the 15-second window balances promptness with the practicalities of train operation and safety protocols.