What is the primary purpose of a 'dead man' or 'alertness' device on a locomotive?

1. To automatically stop the train if the locomotive exceeds the speed limit in the timetable
2. To detect if the engineer has become incapacitated or inattentive and apply the brakes if no response is given
3. To log the number of horn soundings at grade crossings for FRA compliance
4. To monitor the conductor's location within the train via GPS