Application and positioning of railway signals
Signals are used to indicate one or more of the following:
- that the line ahead is clear (free of any obstruction) or blocked.
- that the driver has permission to proceed.
- that points (also called switch or turnout in the US) are set correctly.
- which way points are set.
- the speed the train may travel.
- the state of the next signal.
- that the train orders are to be picked up by the crew.
Signals can be placed:
- at the start of a section of track.
- on the approach to a movable item of infrastructure, such as points/switches or a swingbridge.
- in advance of other signals.
- on the approach to a level crossing.
- at a switch or turnout.
- ahead of platforms or other places that trains are likely to be stopped.
- at train order stations.
‘Running lines’ are usually continuously signalled. Each line of a double track railway is normally signalled in one direction only, with all signals facing the same direction on either line. Where ‘bi-directional’ signalling is installed, signals face in both directions on both tracks (sometimes known as ‘reversible working’ where lines not normally used for bi-directional working). Signals are generally not provided for controlling movements within sidings or yard areas.