Railway signal
From Free net encyclopedia
Template:Mergeto A signal is a mechanical or electrical device that indicates to train drivers information about the state of the line ahead, and therefore whether he or she must stop or may start, or instructions on what speed the train may go.
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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 (also called switch or turnout in the US) 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/switches
- 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/switches, section of track, etc. that they are ahead of.
The term "ahead of" can be confusing, so official UK practice is to use the terms in rear of and in advance of. When a train is waiting at a signal it is "in rear of" that signal and the danger being protected by the signal is "in advance of" the train and signal.
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 facing trains approaching from converging routes at a junction are interlocked so that only one of the two signals can show a "proceed" indication.
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, or pipes supported on rollers (US). 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. A development was that mechanical signals were operated by electric motors that moved the signal arms, and current practice is for mechanical signals to be replaced by colour-light signals.
Signals were originally totally manually-operated, then manually-operated with mechanical checks that prevented them from being operated inappropriately. Later signals were manually set to either "proceed" or "stop", or automatically set to "stop" by devices that detected the presence of a train. Many signals today are fully automatic, with either no manual control or in some cases manual control only when required.
Colour-light signals
- See also: UK railway signalling
Most railways use coloured lights to indicate to the driver of the train what action to be taken. Different colours mean different things and can be used in combination to increase the amount of information that can be indicated. Color-light railway signals display green at the top and red at the bottom, intentionally the opposite of highway signals to avoid mistaking one for the other.
Most railway signals worldwide use a multi-unit signal head with a separate lamp and lens system for each aspect colour, although there are still many "searchlight" type signals in use in the U.S. Reflectors are not used because of the risk of stray sunlight giving the appearance of illumination and thus causing a phantom aspect; signal heads are commonly hooded to help prevent this. Incandescent light bulbs are positioned at the focal point of a lens system usually consisting of a coloured Fresnel lens behind the front transparent lens.
Searchlight signals have one permanently-illuminated lamp, the aspect colour being selected by a moving vane operated by an electric motor that positions the appropriate colour filter in front of the lens. This type of signal does use a (parabolic) reflector and thus lower power lamps and a less directional lens system are used since there is a far greater fraction of the light available to be directed down the tracks towards the driver of the oncoming train. They have the disadvantage of having moving parts in what can be a hostile location for mechanical equipment and thus need regular maintenance. This also means that only one colour can be displayed at a time, though two or more heads can be used.
A variant of this is the Unilens (tm) signal made by Safetran Systems Corporation, which uses a single-lens system, fed by three or four individual halogen lamps with parabolic reflectors behind them. These lamps shine through coloured filters into individual fiber-optic elements, which join together at the focal point of the lens assembly. This makes it possible to show four different colors (usually red/yellow/green/lunar white) from a single signal head, which is impossible for the traditional searchlight mechanism.
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. These are often arranged so that the same aperture is used for whichever colour light is required and are therefore sometimes referred to as modern searchlights.
Mechanically operated signals
Image:Rail semaphore koscierzyna.jpg Image:Semaphore Signal on British Rail Southern Region Redvers.jpg 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 an angle between 30 and 60 degrees to mean 'go.' Upper-quadrant signals (where the "proceed" indication is above the horizontal) have largely replaced lower-quadrant signals (where "proceed" is below horizontal) because a failure that disconnects the mechanism causes the arm to drop into a safe, "stop" position.
Two methods were commonly used to provide a third 'caution' indication, that the train could pass but the next signal is indicating stop. One is the three-position signal, using horizontal for 'stop', 45 degrees for 'caution', and vertical for 'go'. Such signals were widely adopted in the USA after 1908, and there were a few instances in the UK. The other approach, which is the norm in the Britain was to provide a separate distant signal for this purpose - indicating, when 'clear', that the next signal controlled by that signalbox applying to that train is also 'clear.' Initially these signals were not distinguished, relying on the driver to know which was which, but over time these distant first gained a fishtail end, and then in the 1920s changed colour from red to yellow. Ireland however mainained the use of red for both types of signal. Where a distant signal for one signal box is mounted under a home signal for another, a mechanical interlock is used to keep the former at caution when the latter is at danger. (Home signals are now also of two different shapes: square-ended meaning "stop and stay stopped" <when in the stop position>, and pointed <the opposite of a fishtail> meaning "stop and then proceed prepared to stop.")
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 blue (combining with the yellow lamp to give a green light) and red filters mounted on the arm, or blue, yellow and red in the case of three-aspect signals.
Tokens
Traditionally, signalling systems were effective at providing protection where trains were travelling in the same direction. However, on single-line sections, where trains use the same track in both directions for long distances, there was much greater concern about the consequences that would follow from one train failing to stop at a danger signal, and it was felt necessary to provide additional safeguards. The standard approach was for the driver to be given a physical object called a token to authorise him to use a particular stretch of single track. Since there was either only one token, or electrical locking was used to prevent more than one token being released at any one time, for a given section of track the driver could be confident that no train would be coming from the other direction. The token system has now largely been replaced by other systems such as 'Tokenless Block', although the Radio Electronic Token Block system or RETB is still used on remote branch lines in Scotland, Wales, and East Suffolk - in this case, the driver does not physically carry a token, but a 'virtual token' is issued by radio.
International
Railway signals vary widely from country in both the semaphore and colour-light varieties.
- Red on its own means stop
- Yellow and green (and sometimes red) singly or in combination mean caution or clear.
- Colours in adjacent countries can have different meanings.
- Australian railway signalling
- German railway signalling
- Norwegian railway signalling
- UK railway signalling
See also
de:Eisenbahnsignal fr:Signalisation ferroviaire lt:Semaforas nl:Spoorwegsein ja:鉄道信号 pl:Semafor (kolejnictwo) sk:Železničný signál