NSW Railway Signalling

Railway signals are a piece of lineside equipment that provide drivers with a visual indication of the line status ahead, and are used mainly to ensure multiple trains on the same track do not collide. Some may also indicate the status of points ahead, ensuring drivers have sufficient warning to slow the train before going through a turnout.

Most signals in NSW use combinations of colour lights to show the status of the line ahead. This is accomplished using a series of different coloured LED or incandescent lights mounted on a vertical pole or to a tunnel wall. Some of these lights may be attached together into a signal head. This doesn't have much meaning, but usually only one light in each head will be lit.

Most signals in NSW will also have a train stop for additional safety.

Control
The indication (or aspect) displayed on a signal is determined by one of two methods. The position of trains is first determined using track circuits. For automatic signals, the signal circuitry is directly linked to the track circuits and it automatically determines what aspect to display depending on where trains are. The lower head of automatic signals is offset (shifted slightly to the left or right) to tell them apart from controlled signals.

For controlled signals, the track circuits all go back to a control centre, where a signaller will control the indication of the signal.

Aspects
Aspects are combinations of lights displayed on a signal that each have their own meaning. There are two different sets of systems used in NSW.

The double colour light signalling is used in most metropolitan areas, and gives more aspects to allow trains to be closer together. Consequently, more lights are needed in each signal. The signal colour light system is used in country areas, where there are much less train movements.

The general rule of double colour light signalling is that the top head shows what you should do at THIS signal and the bottom head shows what you should do at the NEXT signal. Any small lights below these give supplementary information. For example, green/yellow would mean you can pass this signal and that the you should slow down at the next signal.