Some are common, others are unique to a specific railroad. The most constant signals and the easiest to understand are single-head block signals. Red means stop; green means proceed, and yellow means caution or approach, usually indicating that the next signal is red.
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Depending on the circumstance, if the engineer or conductor claim some extenuating condition, (the red dropped right in front of them) then an investigation would be held, but most of the time the crew admits they ran it, the CTC board will show when and where they got past it, the dispatcher will know, and most likely ...
It could be blocked by another train, by road traffic crossing the line or by people carrying out repairs. Occasionally its because the signal (light) itself is not operating correctly and they fail safe so therefore stop trains. So just to be clear the red light means the driver must stop the train at the light.
Railway signalling principlesThe signal lights usually have four indicators: Green: clear to go. Double yellow: still clear to go, but indicates that the following signal may be a warning. Yellow: warns that the next signal may be red. Red: means stop or danger ahead.
“While tunnels are probably the most obvious obstacle to achieving line of sight, mobile signals are also blocked by mountains, trees, buildings, etc.,” says Val. Even the train itself can block mobile signals, because they are essentially big metal tubes traveling at incredibly high speeds.
Trains use their hornsto signal what the train is doing. Weather they are approaching a crossing, or passing through or stopping at a town or station. 1 long horn blast means approaching station.
Small movements may be acceptable, especially if visibility is trying to be improved between Guard and Dispatcher. Do not dispatch on a red signal, unless the train has overshot the signal. This means whistling, closing the doors, or raising a torch.
A low-voltage electric current is sent between the two rails via a series of relays like the ones in this photo. When a train approaches, the current runs through the train's metal wheels and axles instead of the relays. This short circuit activates the crossing signal.