COLOUR ASPECT SIGNALS

The comparison between railway signals and road traffic lights can be made when we get on to the subject of colour aspect or colour light signals.

Green means go, red means stop and yellow – well, that’s a bit different.

Colour light signals are arguably better suited to trains with higher speeds (such as diesels and electrics) and are simpler and easier to install. Today, they’re closely tied into the block system, working in conjunction with AWS and other safety systems.

There are three main types of colour signal: two-aspect, three-aspect and four-aspect. Two-aspect stop signals show either a red or green light, whereas two-aspect distant signals show either green or yellow lights, and mean ‘be prepared to stop’ (OK, so the yellow/amber isn’t exactly like a road traffic light but it’s pretty close).

If you’re hurtling down the East Coast Main Line at 125mph, you are going to take quite a while to stop and will need plenty of warning (at least a couple of miles). This is where multiple-aspect signals come in.

Three-aspect signals work like a distant signal. If the yellow light shows, it tells the driver to be prepare to stop at the next signal. A four-aspect, more common on high speed lines, add an extra warning. Double-yellows tell the driver to expect the next signal to show a single yellow (and thus the signal after that to be at danger). They enable the driver to adjust the speed accordingly, given the greater stopping distances for high-speed trains.