Signal aspects
Trains need long distances to stop, so the driver needs as much warning of when the next signal is at danger as possible. This is done by displaying a yellow light on the signal. For lines carrying very fast trains this is insufficient warning, so the signal in front of the one displaying the yellow light will display two yellow lights (called double-yellow). This type of signal is a four-aspect signal which displays red, yellow, double-yellow and green. Semaphore signalling uses two different types of signals. A danger signal with a red arm at which the driver must stop when the signal arm is horizontal. A distant signal with a yellow arm is there purely as a warning for when the next signal is at danger.