When must the ATC Start button be activated during MATC operations?

The ATC Start button must be activated every time the train comes to a full stop during MATC operations because this protocol is essential for ensuring that the Automatic Train Control (ATC) system is correctly reset and prepared to resume operation safely. Activation of the ATC Start button serves as a necessary safety measure to confirm that the train engineer is ready and attentive before the train moves again.

This practice is crucial for maintaining a constant awareness of the train's status and ensuring that the ATC system can effectively monitor and control the train's speed and movement. By mandating this action at every full stop, it minimizes the risk of accidents that could result from inadvertent movement after stopping, thereby enhancing the overall safety and operational integrity of the railroad system.

The other options, while relevant to operations, do not specifically address the protocol required for the ATC Start button in relation to train stops, which is the key context for this question.