danger
The stop indication of a signal (usually in the phrase "at danger").
danger: the stop command a signal gives to trains
In railway signalling, a signal is said to be "at danger" when it displays the aspect that instructs a driver to stop immediately. This is the most restrictive state a signal can show, overriding all other considerations. The term is universal across rail systems, though the exact visual representation varies: in Britain, a red light; in some European systems, red combined with a horizontal arm or blade. A signal at danger means the track ahead is either occupied, the points are set unsafely, or a conflicting movement is in progress.
The opposite condition is "at clear" or "off the danger", when a signal displays a permissive aspect (green or yellow depending on line speed and distance to the next signal). Between these states sit intermediate aspects such as yellow or double yellow, which warn of restrictive signals ahead but permit the train to proceed at a reduced speed. Semaphore signals show danger by holding the arm or blade horizontal; colour light signals show it via a red lens; mechanical line-of-sight systems use other conventions. The specific aspect that constitutes danger is always defined in the operating rules for that railway.
A signal held at danger is typically the result of track circuit detection, which senses whether a train occupies a section of track. If a section is occupied, the signal protecting it locks at danger. Points apparatus also locks signals at danger if the points are not correctly set for the required route. Interlocking equipment ensures that no conflicting movements can occur: if a signal protecting one line is at clear, signals protecting crossing or conflicting paths will be held at danger by the mechanical or electronic interlocking machine.
Drivers must always obey a signal at danger and treat it as an absolute stop instruction. Passing a signal at danger is one of the most serious infractions in rail operations and can result in disciplinary action or criminal charges if it causes an accident. Signals can fail or become obscured; in such cases, drivers follow fall-back rules (often treating the signal as at danger unless other evidence confirms it is safe to proceed). The state of a signal at danger is continually monitored by safety systems and recorded in train control logs.