I think a dead man's switch is mentioned:

"The Alerter system is connected directly to the air brake system which functions to provide an automatic full service penalty application of the air brake system, and a power knock out (PC) caused by failure to acknowledge the time out feature usually about 40 seconds. When the Alerter time out has expired, the engineer must acknowledge by tripping the acknowledging switch which will reset the time out feature. The Alerter system is nullified when locomotive brake cylinder pressure of 20 psi is developed in the Independent Application and Release pipe. This also prevents the P2A Application Valve from triggering a service brake application and PC action."