From Longman Dictionary of Contemporary Englishcommunication cordcomˌmuniˈcation ˌcord noun [countable] British English TTTa chain that a passenger can pull to stop a train in an emergency (=a sudden dangerous situation)
Examples from the Corpus
communication cord• Overhead is a communication cord, as in a train.• And on the second train from Oxford they pulled the communication cord to try to avoid us.• I came close to pulling the communication cord.• The man opposite me yanks the communication cord, and we lurch to a halt. 