Deadman’s switch in safety systems is

• A. A device that automatically stops the train after a delay.
• B. A safety device requiring continuous operator input to indicate presence; if released, emergency braking engages.
• C. A door sensor that detects obstructions.
• D. A system that signals the next train to depart.

Answer: (B)

Deadman’s switch is a safety mechanism that ensures the operator remains actively present by requiring continuous input. If the operator releases or can’t provide that input, emergency braking is engaged to stop the train. This prevents a train from continuing under supervision that has been interrupted or lost. The other descriptions don’t fit: stopping automatically after a delay describes a timed stop rather than operator presence; a door sensor for obstructions is a different safety function; signaling the next train to depart is about inter-train communication, not operator presence. So the description that emphasizes continuous input and automatic braking on release is the one that matches a deadman’s switch.