danger signal

• danger-signal

danger signal (plural danger signals)

1. (rail transport) a signal set at danger and showing a red light, a stop signal which a train driver must not pass until it changes.
   • 2005, "Practical Railway Engineering" by Clifford F. Bonnett, Imperial College Press →ISBN

     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.

2. (also figurative) an indication that a potentially bad or dangerous situation could develop.

• (rail) SPAD

• “danger signal” (US) / “danger signal” (UK) in Oxford Dictionaries, Oxford University Press.
• danger signal at OneLook Dictionary Search

• landing gears