Metropolitan Transportation Authority (MTA) Assistant Conductor Practice Test

The Automatic Train Stop (ATS) system is designed as a safety mechanism to prevent train collisions and ensure compliance with operational signals. It is specifically triggered when the train fails to comply with cab signal indications, such as disregarding speed limits or signals to stop. When the ATS detects that the train is exceeding the allowable speed or ignoring a stop signal, it automatically intervenes to bring the train to a halt, thereby enhancing safety on the railway.

In this context, the other options focus on different operational scenarios. Acknowledgment of a speed limit by the engineer reflects the active decision-making role of the operator, rather than a trigger for the ATS system. The act of the train coming into a station is part of normal operation that does not directly trigger the ATS mechanism. Likewise, activation of yard control measures relates to train movements within a yard and does not directly relate to the ATS's primary function. Therefore, the correct answer emphasizes the safety role of the ATS in responding to non-compliance with signals.