Railway signal

A Signal is a mechanical or electrical device that indicates to train drivers information about the state of the line ahead, and therefore whether they must stop or may start, or instructions on what speed they may drive their train.

Network Rail two-aspect electrical railway signal set at danger

Signals are used to indicate one or more of the following:

that the line ahead is clear (free of any obstruction) or blocked
that points are set correctly
which way points are set
that the driver has permission to proceed
the speed the train may travel
the state of the next signal

Signals can be placed:

ahead of points
at the start of a section of track (with block signalling)
ahead of a level crossing
ahead of platforms or other places that trains are likely to be stopped.

Signals are sometimes said to "protect" the points, section of track, etc. that they are placed ahead of.

Control and operation of signals

The earliest signals were directly operated by a signalman on the basis of his knowledge of the line ahead. There was no mechanical check that the signal provided correct information. Later, signals were mechanically connected to the points that they protected, so that the signal could only be set to show a "proceed" indication if the points were in fact set (or set and locked) correctly.

When multiple signals are used to control movements in the same area, the signals will also be connected together to prevent conflicting indications. These signals are said to be "interlocked". For example, two signals at both ends of a station facing trains approaching from opposite directions, are interlocked together so that only one of the two signals can show "proceed" indications at the same time.

A subsequent development was to connect the signals to devices that detected the presence of trains, so that a signal could not show a "proceed" indication when there is a train in the section of track protected by the signal.

Signals were originally controlled by levers situated at the signals, and later by levers grouped together and connected to the signal by wire cables. Often these levers were placed in a special building, known as a signal box (UK) or interlocking tower (US), and they were mechanically interlocked in the signal box. Later developments were electric interlocking and controls instead of mechanical, then software interlocking. Also, mechanical signals were replaced by electrically-operated light signals, although some mechanical signals are also operated by electrical control operating a motor which moves the signal.

Signals were originally totally manually-operated, then manually-operated with mechanical checks that prevented him from operating it inappropriately. Later signals were manually set to either "proceed" or "stop", or automatically set to "stop" by devices that detected the passage of a train. Many signals today are fully automatic, with either no manual control or in some cases manual control only when required.

Electrically operated signals

Most electrically operated railways signals use coloured lights to indicate to the driver of the train what action he should be taking. Different colours mean different things and can be used in combination to increase the amount of information that can be indicated.

Most colour light signalling systems use a set of incandescent light bulbs, each behind a coloured Fresnel lens and in front of a parabolic reflector. This ensures that as much of the light as possible is directed down the tracks towards the driver of the oncoming train. More recently, clusters of LEDs have started to be used in place of the incandescent bulbs, reflectors and lenses. They have a more even colour output, use less power and have a working life of around 10 years, significantly reducing long term costs.

Most railway signals worldwide use one bulb/reflector/lens combination for each colour. However, some signals use one bulb/reflector/lens combination to display multiple colours. They do this by having a rotating colour filter in the signal which can be turned by an electric motor so that it displays the appropriate colour. Of course, this means that only one colour can be displayed at a time.

Electrically operated railway signals in the UK

The railway signalling system used across the majority of the UK railway network uses colour lights to tell the driver the status of the section of track ahead of them. The most common signal type has four lights arranged as follows:

Yellow
Green
Yellow
Red

The combinations of these lights that are used as indication to drivers are called aspects. For a four light signal as above, the main aspects that are used are:

Red - Danger. Next track section occupied. You must stop.
Yellow (bottom only) - warning. The signal after this is at danger. Slow down.
Double yellow (both) - warning. The signal two after this is at danger. Prepare to slow down.
Green - clear. You may proceed.

However, not all railway lines use full four-aspect signalling. Some use a three or even two aspect variant. The three aspect version uses only three colour lights, omitting the top yellow light, to give a Red, Yellow and Green aspect choice. The two aspect version leaves only the Red and the Green aspects, with repeaters (signals that show Green or Yellow) used to give advance warning of a Red.

Unusual signal aspects

There are some more unusual signal aspects in use in places across the UK.

Flashing (double) yellow/green - a flashing yellow, double yellow or green aspect indicates that the aspect is being used for approach control. That is, to indicate to the driver to slow the train down in time for a speed limit that they will be approaching as a result of them diverging from their current track. These aspects usually automatically clear to one of the traditional aspects once the train is close to the signal.

Shunting signals, feathers and other indicators

Shunting signals are much smaller and are laid out in a triangular formation. They consist of either: two red lights, or one red and one white light. Two red lights means you may not proceed. A red and a white light means the same as two reds. The proceed aspect consists of two white lights, the red light(s) having been extinguished. If a shunting signal shows one yellow and one white light, (or two yellows), you may proceed with caution as far as it is safe to do so, or to the limit of shunt.

Feathers are used to indicate to the driver which route they are taking at the proceeding junction. They consist of a row, column or diagonal of white lights. Where more than one diverging route is possible, a combination of rows, columns and diagonals can be used. When the primary non-diverging route is set, the feather is not illuminated. When a diverging route is set, the respective feather is illuminated.

Route indicators are alphanumeric displays next to a signal to display either the line or platform to which the train has been routed. They are typically white lamps illuminated in a dot-matrix fashion to give an alphanumeric display.

AWS

Automatic Warning System (AWS) is a system linked to the signalling systems to warn the driver about the aspect of the next signal. These warnings are typically 200 yards before the signal. Information about the signal aspect is conveyed by magnetism to the moving train through boxes fixed in the middle of the track, known as AWS ramps.

If the signal being approached is set to clear, the AWS will set the visual indicator to off and sound a bell. This lets the driver know that the next signal is set to clear and that the AWS system is working. If the signal being approached is set to any danger aspect (red, yellow, double yellow etc.), the AWS will set the visual indicator to on and sound a horn. The horn will continue to sound until the driver acknowledges the AWS warning by pushing a button. If the driver fails to cancel the warning after a period of a few seconds, a full emergency brake application is made to bring the train to a halt.

The AWS was first used on the Great Western line out of Paddington in the days of steam. Its potential to stop accidents was quickly seen and it was eventually adopted nationwide. AWS also works with mechanical signalling methods. The latest system which is being used to augment AWS is the Train Protection & Warning System.

Mechanically operated signals

Mechanical semaphore in Koscierzyna

Prior to the introduction of electric signals, mechanically operated semaphore signals were widely used, operated by a wire rope from the signal box (and later often by electricity). They were so named after their resemblance to the Semaphore flags.

The signal consisted of an arm mounted on a tall post, mast, or gantry, or sometimes projecting from a building, with a horizontal arm meaning 'stop', and an arm raised or lowered at a 45 to 60 degree angle to mean 'go'. Three positions were used in some countries, at 0, 45 and 90 degrees, the intermediate position indicating that the train could pass but that the next signal was indicating stop; such signals were widely adopted in the USA after 1908. A separate signal was used for this purpose in other countries; for example Britain adopted a separate distant signal for this purpose on high speed lines or when the visibility distance was reduced, coloured yellow rather than the red of ordinary signals.

While some signals used a lowered arm (lower-quadrant signal) to indicate that the signal could be passed, these were generally converted to the safer raised arm (upper-quadrant signal), which enabled the signal to fail safe in the case of a mechanical problem.

Although not officially sanctioned, in Britain the practice of raising a signal slowly from 'stop' to 'go' indicated that the status of the line ahead was uncertain, and that the train might proceed with great caution.

For night operation, an oil (or later an electric) lamp was lit on the post, its colour changing as the arm moved by means of green and red filters mounted on the arm, or green yellow and red in the case of three-aspect signals.

Tokens

Before signals were used, a driver was given a physical object called a token to authorise him to use a particular stretch of single track. Since there was only one token for each length of track, the driver could be confident that another train would not meet him from the other direction. The token system has now largely been replaced, although an "electronic token" is still used on remote branch lines in Scotland, Wales, and East Suffolk.