One of the best known examples is the ' Dead Man's Handle ' as used traditionally on many Diesel and Electric trains .
Driver has to keep a handle or treadle depressed whilst driving . If driver releases pressure then first a warning siren sounds , then a partial brake application is automatically made and then a full brake application if driver does not respond .
Many variants . There is normally a procedure which allows driver to release handle/ treadle briefly without triggering the alarm and braking so as to be able to do necessary tasks away from the driving position .
That is the traditional system . Modern trains have much more complex integrated safety systems .