danger signal

noun2

1. •
   A signal set at danger and showing a red light or other stop indication, which a train driver must not pass until it changes or until manually cleared past the signal by a dispatcher.

   • On a train travelling at say 40 miles per hour a driver would have to react quite quickly to bring his train to a halt at a danger signal even if he could see that signal half a mile away. — 2005
2. •
   (figurative) An indication that a potentially bad or dangerous situation could develop.