What must trains do when receiving a message from a trackside warning detector?

When receiving a message from a trackside warning detector, trains must acknowledge the transmission over the radio. This acknowledgment serves a critical safety function. It ensures that the train crew is aware of the detector's message, which may contain important information regarding track conditions, potential hazards, or other operational directives. By acknowledging the message, the train crew confirms that they have received and understood the information, which is essential for maintaining effective communication between the train and dispatchers, especially in crucial situations where immediate action or awareness may be needed.

The process of acknowledgment is not just a formality; it contributes to overall operational safety and efficiency by ensuring that all parties involved are informed and can act accordingly. It helps prevent misunderstandings or oversight that could lead to unsafe conditions or accidents on the railways.