A railway signal is a visual display device that conveys instructions or provides warning of instructions regarding the driver’s authority to proceed. The driver interprets the signal's indication and acts accordingly. Typically, a signal might inform the
driver of the speed at which the train may safely proceed or it may instruct the driver to stop.
Application and positioning of signals
![Rail-routeindicator](https://upload.wikimedia.org/wikipedia/commons/1/17/Rail-routeindicator.jpg)
Originally, signals displayed simple stop or 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 they were approaching a signal which might require a stop. This allowed for an overall increase in speed, since train drivers no longer had to drive at a speed within sighting distance of the stop signal.
Under
timetable and train order
A train order is "an order issued by or through a proper railway official to govern the movement of trains". Train order operation is the system by which trains are safely moved by train orders. It is distinguished from other forms of train opera ...
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 order
A train order is "an order issued by or through a proper railway official to govern the movement of trains". Train order operation is the system by which trains are safely moved by train orders. It is distinguished from other forms of train opera ...
s are to be picked up by the crew
Early signal systems
![Einfahrsignal Friedrichsdorf Hp2 Zs3v](https://upload.wikimedia.org/wikipedia/commons/2/2d/Einfahrsignal_Friedrichsdorf_Hp2_Zs3v.jpg)
Signals can be placed:
* At the start of a
section
Section, Sectioning or Sectioned may refer to:
Arts, entertainment and media
* Section (music), a complete, but not independent, musical idea
* Section (typography), a subdivision, especially of a chapter, in books and documents
** Section sign ...
of track
* On the approach to a movable item of infrastructure, such as points or switches or a
swingbridge
A swing bridge (or swing span bridge) is a movable bridge that has as its primary structural support a vertical locating pin and support ring, usually at or near to its center of gravity, about which the swing span (turning span) can then pi ...
* In advance of other signals
* On the approach to a
level crossing
A level crossing is an intersection where a railway line crosses a road, Trail, path, or (in rare situations) airport runway, at the same level, as opposed to the railway line crossing over or under using an Overpass#Railway, overpass ...
* At a switch or turnout
* Ahead of platforms or other places that trains are likely to be stopped
* At
train order station
A train order is "an order issued by or through a proper railway official to govern the movement of trains". Train order operation is the system by which trains are safely moved by train orders. It is distinguished from other forms of train opera ...
s
'Running lines' are usually continuously signalled. Each line of a
double track
A double-track railway usually involves running one track in each direction, compared to a single-track railway where trains in both directions share the same track.
Overview
In the earliest days of railways in the United Kingdom, most lin ...
railway is normally signalled in one direction only, with all signals facing the same direction on either line. Where
bidirectional signalling is installed, signals face in both directions on both tracks (sometimes known as 'reversible working' where lines are not normally used for bidirectional working). Signals are generally not provided for controlling movements within
sidings or yard areas.
Aspects and indications
![Rail-semaphore-signal-Dave-F](https://upload.wikimedia.org/wikipedia/commons/7/7b/Rail-semaphore-signal-Dave-F.jpg)
Signals have ''aspects'' and ''indications''. The ''aspect'' is the visual appearance of the signal; the ''indication'' is the meaning. In American practice the indications have conventional names, so that for instance "Medium Approach" means "Proceed at not exceeding medium speed; be prepared to stop at next signal". Different railroads historically assigned different meanings to the same aspect, so it is common as a result of mergers to find that different divisions of a modern railroad may have different rules governing the interpretation of signal aspects. For example, stop aspect refers to any signal aspect that does not allow the driver to pass the signal.
![Finnish distant signal displaying Expect Stop](https://upload.wikimedia.org/wikipedia/commons/e/e5/Finnish_distant_signal_displaying_Expect_Stop.jpg)
Unlike a typical
traffic light
Traffic lights, traffic signals, or stoplights – known also as robots in South Africa are signalling devices positioned at intersection (road), road intersections, pedestrian crossings, and other locations in order to control flows of traf ...
, where both the position and colour of the light when lit convey the same indication, in a signal head with multiple lamps, both the colour and position of lit lamps are necessary to interpret the aspect of the signal.
Signals control motion past the point at which the signal stands and into the next section of track. They may also convey information about the state of the next signal to be encountered. Signals are sometimes said to "protect" the points or 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.
In North American practice, a distinction must be made between ''absolute'' signals, which can display a "Stop" (or "Stop and Stay") indication, and ''permissive'' signals, which display a "Stop & Proceed" aspect. Furthermore, a permissive signal may be marked as a ''Grade Signal'' where a train does not need to physically stop for a "Stop & Proceed" signal, but only decelerate to a speed slow enough to stop short of any obstructions.
Interlocking
In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junction (rail), junctions or crossings. The signalling appliances and Track (rail transpor ...
('controlled') signals are typically absolute, while automatic signals (i.e. those controlled through track occupancy alone, not by a signalman) are usually permissive.
Drivers need to be aware of which signals are automatic. In current British practice for example, automatic signals have a white rectangular plate with a black horizontal line across it. In US practice a permissive signal typically is indicated by the presence of a number plate. In the Australian states of New South Wales, Victoria and South Australia, as well as New Zealand, a permissive signal has the lower set of lights offset (usually to the right) from the upper lights; in Victoria and New Zealand, an absolute signal displaying a red or white "A" light is also treated as a permissive signal. Some types of signal display separate permissive and absolute stop aspects. In Germany, the rules which apply to the respective signal are indicated by a vertical plate on the signal's post ().
Operating rules normally specify that a signal with an abnormality, such as one with an extinguished lamp or an entirely dark signal, must be interpreted as the most restrictive aspectgenerally "Stop" or "Stop and Proceed".
Signal forms
Signals differ both in the manner in which they display aspects and in the manner in which they are mounted with respect to the track.
Mechanical signals
![Rail semaphore koscierzyna cropped](https://upload.wikimedia.org/wikipedia/commons/e/ed/Rail_semaphore_koscierzyna_cropped.jpg)
The oldest forms of signal displays their different indications by a part of the signal being physically moved. The earliest types comprised a board that was either turned face-on and fully visible to the driver, or rotated away so as to be practically invisible. These signals had two or at most three positions.
Semaphore signals were developed in France at the end of the 18th century, before being later adopted by the railways. The first railway semaphore was erected by
Charles Hutton Gregory
Sir Charles Hutton Gregory (14 October 1817 – 10 January 1898) was an English people, English civil engineer. He was president of the Institution of Civil Engineers between December 1867 and December 1869.
Charles was the son of Oli ...
on the
London and Croydon Railway
The London and Croydon Railway (L&CR) was an early railway in England. It opened in 1839 and in February 1846 merged with other railways to form the London Brighton and South Coast Railway (LB&SCR).
Origins
The Croydon line and other railways
Th ...
(later the Brighton) at
New Cross Gate
New Cross is an area in south east London, England, south-east of Charing Cross in the London Borough of Lewisham and the SE14 postcode district. New Cross is near St Johns, Telegraph Hill, Nunhead, Peckham, Brockley, Deptford and Greenwi ...
, southeast London, in 1841. It was similar in form to the
optical telegraph
An optical telegraph is a line of stations, typically towers, for the purpose of conveying textual information by means of visual signals. There are two main types of such systems; the semaphore telegraph which uses pivoted indicator arms and ...
s then being replaced on land by the
electric telegraph
Electrical telegraphs were point-to-point text messaging systems, primarily used from the 1840s until the late 20th century. It was the first electrical telecommunications system and the most widely used of a number of early messaging systems ...
. Gregory's installation was inspected and approved for the
Board of Trade
The Board of Trade is a British government body concerned with commerce and industry, currently within the Department for International Trade. Its full title is The Lords of the Committee of the Privy Council appointed for the consideration of ...
by Major-General
Charles Pasley
General Sir Charles William Pasley (8 September 1780 – 19 April 1861) was a British soldier and military engineer who wrote the defining text on the role of the post-American Revolution British Empire: ''An Essay on the Military Policy and Ins ...
. Pasley had invented a system of optical telegraphy through semaphores in 1822 for the British military, and appears to have suggested to Gregory the application of the semaphore to railway signaling. The semaphore was afterwards rapidly adopted as a fixed signal nearly universally. Disc signals, such as those made by the
Hall Signal Company
The Hall Signal Company was an American manufacturer of Railway Signalling, railway signaling equipment in the 19th and 20th centuries. Hall's equipment was widely used by American railroad companies. The company's founder, William Phillips Hall, ...
, were sometimes used, but semaphores could be read at much longer distances. The invention of the
electric light
An electric light, lamp, or light bulb is an electrical component that produces light. It is the most common form of artificial lighting. Lamps usually have a base made of ceramic, metal, glass, or plastic, which secures the lamp in the soc ...
, which could be made brighter than
oil lamp
An oil lamp is a lamp used to produce light continuously for a period of time using an oil-based fuel source. The use of oil lamps began thousands of years ago and continues to this day, although their use is less common in modern times. Th ...
s and hence visible both by night and day, resulted in the development of
position light signal
A railway signal is a visual display device that conveys instructions or provides warning of instructions regarding the driver’s authority to proceed. The driver interprets the signal's indication and acts accordingly. Typically, a signal mi ...
s and colour-light signals at the beginning of the 20th century, which gradually displaced semaphores. A few remain in modern operations in the United Kingdom.
Mechanical signals may be operated manually, connected to a lever in a signal-box, by electric motors, or hydraulically. The signals are designed to be
fail-safe
In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safe ...
so that if power is lost or a linkage is broken, the arm will move by gravity into the horizontal position.
In the U.S., semaphores were employed as train order signals, with the purpose of indicating to engineers whether they should stop to receive a
telegraphed
Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas p ...
order, and also as simply one form of block signalling.
Colour light signals
![German HP signals](https://upload.wikimedia.org/wikipedia/commons/f/f0/German_HP_signals.jpg)
The introduction of electric
light bulb
An electric light, lamp, or light bulb is an electrical component that produces light. It is the most common form of artificial lighting. Lamps usually have a base made of ceramic, metal, glass, or plastic, which secures the lamp in the soc ...
s made it possible to produce colour light signals which were bright enough to be seen during daylight, starting in 1904.
![Romanian signal at Ploiesti Vest](https://upload.wikimedia.org/wikipedia/commons/d/d8/Romanian_signal_at_Ploiesti_Vest.jpg)
The ''signal head'' is the portion of a colour light signal which displays the aspects. To display a larger number of indications, a single signal might have multiple signal heads. Some systems used a single head coupled with auxiliary lights to modify the basic aspect.
Colour light signals come in two forms. The most prevalent form is the ''multi-unit'' type, with separate lights and lenses for each colour, in the manner of a
traffic light
Traffic lights, traffic signals, or stoplights – known also as robots in South Africa are signalling devices positioned at intersection (road), road intersections, pedestrian crossings, and other locations in order to control flows of traf ...
. Hoods and shields are generally provided to shade the lights from sunlight which could cause false indications.
![SearchlightSignalMechanism](https://upload.wikimedia.org/wikipedia/commons/e/e7/SearchlightSignalMechanism.JPG)
Searchlight signals were the most often used signal type in the U.S. until recently. In these, a single
incandescent light bulb
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...
is used in each head, and either an
A.C. or
D.C. relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
mechanism is used to move a coloured spectacle (or "roundel") in front of the lamp. In this manner, gravity (fail safe) returns the red roundel into the lamp's optical path. In effect, this mechanism is very similar to the colour light signal that is included in an electrically operated semaphore signal, except that the omission of the semaphore arm allows the roundels to be miniaturized and enclosed in a weatherproof housing. Widely used in the U.S from
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
onward, searchlight signals have the disadvantage of having moving parts which may be deliberately tampered with. This had led to them becoming less common during the last fifteen to twenty years when vandalism began to render them vulnerable to false indications.
However, in some other countries, such as on the
Italian railways
The Italian railway system is one of the most important parts of the infrastructure of Italy, with a total length of of which active lines are 16,723 km. The network has recently grown with the construction of the new high-speed rail netw ...
(
FS) as from th
''Regolamento Segnali'' they are still the standard colour light signal albeit with new installations being as outlined below.
More recently, clusters of
LEDs
A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (cor ...
have started to be used in place of the incandescent lamps, reflectors and lenses. These use less power and have a purported working life of ten years, but this may not in reality be the case.
Operating rules generally dictate that a dark signal be interpreted as giving the most restrictive indication it can display (generally "stop" or "stop and proceed"). Many colour light systems have circuitry to detect such failures in lamps or mechanism.
Position light signals
![OVERBROOK 18R-Approach-Medium](https://upload.wikimedia.org/wikipedia/commons/e/e0/OVERBROOK_18R-Approach-Medium.jpg)
A position light signal is one where the position of the lights, rather than their colour, determines the meaning. The aspect consists solely of a pattern of illuminated lights, which are all of the same colour. In many countries, small position light signals are used as shunting signals, while the main signals are of colour light form. Also, many tramway systems (such as the Metro of Wolverhampton) use position light signals.
Colour-position signals
![CSX BAILEY-Marc-1-CPL-Dwarf-STOP](https://upload.wikimedia.org/wikipedia/commons/d/d8/CSX_BAILEY-Marc-1-CPL-Dwarf-STOP.jpg)
A system combining aspects of the colour and position systems was developed on the
Baltimore and Ohio Railroad
The Baltimore and Ohio Railroad was the first common carrier railroad and the oldest railroad in the United States, with its first section opening in 1830. Merchants from Baltimore, which had benefited to some extent from the construction of ...
(B&O) in 1920 and was patented by L.F. Loree and F.P. Patenall. It is similar to the position light system with the central light removed and the resulting pairs of lights colored in correspondence to the angle they make: green for the vertical pair, amber for the right diagonal pair, and red for the horizontal pair. An additional pair, colored "lunar white", may be added on the other diagonal for restricting indications. Speed signalling is indicated not by additional signal heads, but by a system of white or amber "orbital" lights placed in one of six positions above and below the main head. The position above or below indicates the current speed, while the left-to-right position indicates the speed at the next signal (full, medium, or slow in both cases). Dwarf signals have the same aspects as full-sized signals. One of the advantages claimed for the system is that burned-out bulbs produce aspects which can be interpreted unambiguously as either the intended indication (for the main head) or as a more restrictive indication (for the orbitals—if only the central head is lit, the indication is either slow or restricting).
Colour position lights (CPLs) were first installed as a pilot on the
Staten Island Railway
The Staten Island Railway (SIR) is a rapid transit line in the New York City borough (New York City), borough of Staten Island. It is owned by the Staten Island Rapid Transit Operating Authority (SIRTOA), a subsidiary of the Metropolitan Trans ...
in New York City, at the time a B&O subsidiary; they were also applied to the
Chicago and Alton Railroad
The Alton Railroad was the final name of a railroad linking Chicago to Alton, Illinois; St. Louis, Missouri; and Kansas City, Missouri. Its predecessor, the Chicago and Alton Railroad , was purchased by the Baltimore and Ohio Railroad in 1931 an ...
when the latter was under B&O control, as well as on the B&O itself. With the disappearance of the B&O into CSX they have been gradually replaced with NORAC color light signals.
Signal mounting
Lineside signals need to be mounted in proximity to the track which they control.
Post mounting
When a single track is involved, the signal is normally mounted on a ''post'' or ''mast'' which displays the arm or signal head at some height above the track, in order to allow it to be seen at a distance. The signal is normally placed on the
engine driver
A train driver, engine driver, engineman or locomotive driver, commonly known as an engineer or railroad engineer in the United States and Canada, and also as a locomotive handler, locomotive operator, train operator, or motorman, is a pers ...
's side of the track.
Gantry mounting
![SR signals from cab of standard five](https://upload.wikimedia.org/wikipedia/commons/c/cb/SR_signals_from_cab_of_standard_five.jpg)
When multiple tracks are involved, or where space does not permit post mounting, other forms are found. In double track territory one may find two signals mounted side by side on a bracket which itself is mounted on a post. The left hand signal then controls the left-hand track, and the right signal the right-hand track. A ''gantry'' or ''signal bridge'' may also be used. This consists of a platform extending over the tracks; the signals are mounted on this platform over the tracks they control.
Ground mounting
![Railway signal at Utrecht Centraal](https://upload.wikimedia.org/wikipedia/commons/a/a9/Railway_signal_at_Utrecht_Centraal.jpg)
In some situations or places, such as in tunnels, where there is insufficient room for a post or gantry, signals may be mounted at ground level. Such signals may be physically smaller (termed ''dwarf signals'').
Rapid transit
Rapid transit or mass rapid transit (MRT), also known as heavy rail or metro, is a type of high-capacity public transport generally found in urban areas. A rapid transit system that primarily or traditionally runs below the surface may be c ...
systems commonly use only dwarf signals due to restricted space. In many systems, dwarf signals are only used to display 'restrictive' aspects such as low speed or shunt aspects, and do not normally indicate 'running' aspects.
Other
Occasionally, a signal may be mounted to a structure such as a
retaining wall
Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides.
Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to ...
, bridge abutment, or overhead
electrification
Electrification is the process of powering by electricity and, in many contexts, the introduction of such power by changing over from an earlier power source.
The broad meaning of the term, such as in the history of technology, economic histor ...
support.
Filaments
Electric lamps for railway signals are often fitted with twin
filaments, so that if one burns out, the other keeps the signal lit. A more complicated version of this, such as in the SL35 lamp, a filament changeover relay is fitted in series with the first filament, where if the first filament burns out, the relay drops and lights the second filament. This filament fail relay also activates an alarm in the signal box.
Lamp proving
When lamps fail, this can result in aspects that are less restrictive (high speed) than when the lamps are correctly lit. This is potentially dangerous.
For example, in UK practice, if a white "feather" indicator fails, the low speed feather combined with a green light, which is low speed, becomes a green light on its own, which is high speed. A lamp proving relay would detect the reduction in current when more than two lamps are not working in a failed feather indicator, and prevent the green from showing. It can also display an indication on the signaller's panel.
Due to this possibility, most signals are configured to be
failsafe
In engineering, a fail-safe is a design feature or practice that in the event of a specific type of failure, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safe ...
.
For example, a flashing aspect can be used to display a less restrictive signal. In this case, if the relay that controls the flashing fails, the signal becomes more restricting. A flashing yellow, in Canada and the United States, at least on the top or middle heads if there is more than one light, means advance clear to stop (or alternatively advance approach) which means expect the next signal to be clear but not the one after. A solid yellow means clear to stop (or alternatively approach) which means expect the next signal to be stop.
Control and operation of signals
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
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
(UK) or interlocking tower (US), and eventually they were mechanically
interlocked to prevent the display of a signal contrary to the alignment of the switch points. Automatic traffic control systems added
track circuit
A track circuit is an electrical device used to prove the absence of a train on rail tracks to signallers and control relevant signals. An alternative to track circuits are axle counters.
Principles and operation
The basic principle behind t ...
s to detect the presence of trains and alter signal aspects to reflect their presence or absence.
Cab signalling
Some locomotives are equipped to display ''cab signals''. These can display signal indications through patterns of lights in the locomotive cab, or in simple systems merely produce an audible sound to warn the driver of a restrictive aspect. Occasionally, cab signals are used by themselves, but more commonly they are used to supplement signals placed at lineside. Cab signalling is particularly useful on
high speed railways. In the absence of lineside signals, fixed markers may be provided at those places where signals would otherwise exist, to mark the limit of a movement authority.
Signalling power
![Delhi metro 1](https://upload.wikimedia.org/wikipedia/commons/7/7a/Delhi_metro_1.jpg)
Usually, signals and other equipment (such as track circuits and level crossing equipment), are powered from a low voltage supply. The specific voltage varies with the country and equipment used. The reason behind this is that the low voltage allows easy operation from storage batteries and indeed, in some parts of the world (and previously in many more locations, before the widespread adoption of electricity), batteries are the primary power source, as mains power may be unavailable at that location.
In urban built-up areas, the trend is now to power signal equipment directly from mains power, with batteries only as backup.
See also
*
Gantry
A gantry is an overhead bridge-like structure supporting equipment such as a crane, signals, or cameras.
Devices and structures
*Gantry (medical), cylindrical scanner assembly used for medical 3D-imaging or treatment
*Gantry (transport), an over ...
*
Highball Signal
Highball Signal is a historic railroad signal located in Delmar, Delaware, United States. It is a white sphere mounted on a pole and located next to the railroad tracks to signal permission for a train to proceed at full speed, if the ball was at ...
*
North American railroad signals
North American railroad signals generally fall into the category of multi-headed electrically lit units displaying ''speed-based'' or ''weak route signaling''. Signals may be of the searchlight, color light, position light, or color position lig ...
*
Railway signalling
Railway signalling (), also called railroad signaling (), is a system used to control the movement of railway traffic. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormou ...
*
Train protection system
A train protection system is a railway technical installation to ensure safe operation in the event of human error.
Development
Train stops
The earliest systems were train stops, as still used by the New York City Subway, the Toronto subway, t ...
*
Train speed optimization {{Unsourced, date=November 2019
Train speed optimization, also known as Zuglaufoptimierung, is a system that reduces the need for trains to brake and accelerate, resulting in smoother and more efficient operation.
While train speed optimization n ...
References
* Armstrong, John (1957). "All About Signals" (Two-article series)
''Trains'' Magazine June and July 1957.
* Calvert, J.B
2007-05-04.
* Kichenside, G. and Williams, A., (1998), ''Two Centuries of Railway Signalling'', Oxford Publishing Co.,
* Solomon, B., (2010), ''Railroad Signaling'', Voyageur Press,
* Vanns, M.A., (1995), ''Signalling in the Age of Steam'', Ian Allan,
Notes
External links
Signal Page– Worldwide railway signalling info and references
{{Authority control
Rail infrastructure
lt:Semaforas
pt:Sinalização urbana