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.
Left: Red LED lights illuminate when a train is detected. The pot (short for potentiometer, the small, adjustable Philips screwhole on the circuit board) can be adjusted with a screwdriver to alter the amount of time the sequence takes to go from red to green.