Dead-man's vigilance device
A dead-man's vigilance device (also called the driver vigilance device or DVD for short[1]) is a railroad safety device that operates in the case of incapacitation of the driver. It is a hybrid between a dead-man's switch and a vigilance control. The main safety failing with the basic dead-man's control system is the possibility of the operating device being permanently held in position, either deliberately or accidentally. The dead-man's vigilance device was developed to detect this condition by requiring that the dead-man's device be released momentarily and re-applied at timed intervals.

Modern practice
Modern locomotive practice is to incorporate the dead-man's and vigilance functions under the control of the alerter or the event recorder.[2]
Warning and braking
If the timer period is beginning to expire, a visual and audible warning is given. If the operator fails to acknowledge the warning, a penalty brake application results.
Accidents due to insufficient vigilance control
References
- "Driver Vigilance Devices – Systems Review (T024)". RSSB.co.uk. Archived from the original on 2018-08-04. Retrieved 2018-08-04.
- "Bach-Simpson". Wabtec.