detonator

noun

1. A device used to detonate an explosive device etc.
2. (UK)A small explosive device attached to the railhead to provide an audible warning when a train passes over it.

   “Warning messages were relayed to the signalmen at Slough East and Dolphin Junction boxes with such celerity that the up main signals were replaced to danger in front of the express before it finally stopped. The signalman at Dolphin Junction just had time to replace the down main signals and pull the emergency lever to lay detonators in front of the fast approaching 7 p.m. Paddington-Reading train.”

3. (archaic)Any explosive whose action is practically instantaneous.
4. (obsolete)A gun fired by a percussion cap.