Railway Signal - Application and Positioning of Signals

Application and Positioning of Signals

Originally, signals displayed simple stop/proceed indications. As traffic density increased, this proved to be too limiting and refinements were added. One such refinement was the addition of distant signals on the approach to stop signals. The distant signal gave the driver warning that he was approaching a signal which might require a stop. This allowed for an increase in speed, since trains no longer needed to be able to stop within sighting distance of the stop signal.

Under timetable and train order operation, the signals did not directly convey orders to the train crew. Instead, they directed the crew to pick up orders, possibly stopping to do so if the order warranted it.

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 are not normally used for bi-directional working). Signals are generally not provided for controlling movements within sidings or yard areas.