danger signal
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.
danger signal: the red light that means stop, full stop
A danger signal is a railway signal displaying a stop indication, usually a red light or equivalent, that forbids a train from proceeding. The driver must bring the train to a complete halt before the signal post and remain stopped until either the signal clears automatically or a signaller or dispatcher manually authorizes passage. This is the primary mechanism enforcing safe spacing and preventing collisions on a line.
The signal may be set to danger either because another train occupies the section ahead, because track work is in progress, or because the signalling system has detected a fault. Modern danger signals are typically three-position (danger, caution, clear) or four-position signals with colour-light heads. Older mechanical semaphore arms show danger when horizontal or at a steep angle. The signal remains at danger until the signaller receives confirmation that the line ahead is safe, then pulls the lever or enters the clearance into a computerized interlocking system.
Absolute and permissive signals
An absolute danger signal must never be passed without explicit permission from the signaller. A permissive danger signal, found on some branch lines or sidings, may in certain circumstances be passed at reduced speed and with caution, but only after the driver has stopped and received clear authorization. This distinction is critical: treating a permissive signal as absolute, or vice versa, is a serious operating error.
Drivers are trained to recognize danger signals by their position, colour, and any accompanying audible warnings. On modern trains equipped with automatic train protection (ATP) or positive train control (PTC), the system will automatically apply the brakes if the driver attempts to pass a danger signal, providing a safety layer independent of human judgment. This redundancy is essential on high-speed and heavily trafficked lines.
The term 'danger signal' reflects the binary safety logic of railway operations: a signal either permits movement or it does not. Any ambiguity could be lethal. This is why railway signals are among the most regulated and standardized equipment in industrial use, with strict rules governing their installation, maintenance, and operation across each national network.