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[[File:Freightliner 66549 gives way to a FGW HST.jpg|thumb|right|A [[British Rail Class 66|Class 66]] locomotive (right) is waiting at a red signal while a [[First Great Western Railway]] (train operating company)|Firstnow Great Western]] Railway) passenger train (left) crosses its path at a junction.]]
 
'''Railway signalling''' ({{abbr|BE|British English}}), also calledor '''railroad signaling''' ({{abbr|AE|American English}}), is a system used to control the movement of [[railway]] traffic. Trains move on fixed [[Rail tracks|rails]], making them uniquely susceptible to [[collision]]. This susceptibility is exacerbated by the enormous weight and inertia of a train, which makes it difficult to quickly stop when encountering an obstacle. In the [[United Kingdom|UK]], the [[Regulation of Railways Act 1889]] introduced a series of requirements on matters such as the implementation of interlocked block signalling and other safety measures as a direct result of the [[Armagh rail disaster]] in that year.
 
Most forms of [[Signalling block systems|train control]] involve movement authority being passed from those responsible for each section of a rail network (e.g. a [[Signalman (rail)|signalman]] or [[stationmaster]]) to the train crew. The set of rules and the physical equipment used to accomplish this determine what is known as the ''method of working'' (UK), ''method of operation'' (US) or ''safeworkingsafe-working'' (Aus.). Not all these methods require the use of physical [[Railway signal|signals]], and some systems are specific to [[Single track (rail)|single -track]] railways.
 
The earliest rail cars were hauled by horses or mules. A mounted flagman on a horse preceded some early trains. Hand and arm signals were used to direct the "train drivers". Foggy and poor-visibility conditions later gave rise to flags and lanterns. Wayside signalling dates back as far as 1832, and used elevated flags or balls that could be seen from afar.
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== Block signalling ==
{{Main|Signalling block systems}}
[[ImageFile:Rail-semaphore-signal-Dave-F.jpg|thumb|upright|British lower-quadrant semaphore stop signal (absolute) with subsidiary arm (permissive) below]]
[[ImageFile:Suojastusopastin Km547.jpg|thumb|upright|A block signal and a 130 km/h speed limit at kilometer post 547 in [[Iisalmi]], Finland]]
 
Trains cannot collide with each other if they are not permitted to occupy the same section of track at the same time, so railway lines are divided into sections known as ''blocks''. In normal circumstances, only one train is permitted in each block at a time. This principle forms the basis of most railway safety systems. Blocks can either be fixed (block limits are fixed along the line) or moving blocks (ends of blocks defined relative to moving trains).<ref name=":0">{{Cite web |url=https://www.era.europa.eu/sites/default/files/filesystem/ertms/ccs_tsi_annex_a_-_mandatory_specifications/set_of_specifications_2_etcs_b3_mr1_gsm-r_b1/index003_-_subset-023_v310.pdf |title=Subset-023 Glossary of Terms and Abbreviations (issue 3.1.0) |publisher=ERTMS USERS GROUP |date=2014-05-12 |website=era.europa.eu |accessarchive-url=https://web.archive.org/web/20200807143950/https://www.era.europa.eu/sites/default/files/filesystem/ertms/ccs_tsi_annex_a_-_mandatory_specifications/set_of_specifications_2_etcs_b3_mr1_gsm-r_b1/index003_-_subset-023_v310.pdf |archive-date=2020-08-0507 |access-date=2024-02-17 }}</ref>
 
=== History of block signalling ===
On double tracked railway lines, which enabled trains to travel in one direction on each track, it was necessary to space trains far enough apart to ensure that they could not collide. In the very early days of railways, men (originally called 'policemen', which is the origin of UK signalmen being referred to as "bob", "bobby" or "officer", when train-crew are speaking to them via a signal telephone) were employed to stand at intervals ("blocks") along the line with a [[stopwatch]] and use hand signals to inform train drivers that a train had passed more or less than a certain number of minutes previously. This was called "time interval working". If a train had passed very recently, the following train was expected to slow down to allow more space to develop.
 
The watchmen had no way of knowing whether a train had cleared the line ahead, so if a preceding train stopped for any reason, the crew of a following train would have no way of knowing unless it was clearly visible. As a result, accidents were common in the early days of railways. With the invention of the [[electrical telegraph]], it became possible for staff at a station or [[signal box]] to send a message (usually a specific number of rings on a [[bellBell (instrument)|bell]]) to confirm that a train had passed and that a specific block was clear. This was called the "[[Absolute block signalling|absolute block system]]".
 
Fixed mechanical signals began to replace hand signals from the 1830s. These were originally worked locally, but it later became normal practice to operate all the signals on a particular block with levers grouped together in a signal box. When a train passed into a block, a [[signalmanSignalman (rail)|signalman]] would protect that block by setting its signal to 'danger'. When an 'all clear' message was received, the signalman would move the signal into the 'clear' position.
 
[[ImageFile:Gantry of Sri Lanka semaphore signals,main line,Sri Lanka.JPG|thumb|upright|Railway infrastructure on the hill-country main line, [[Sri Lanka]], including a gantry of [[Railway semaphore signal|semaphore signals]]]]
 
The absolute block system came into use gradually during the 1850s and 1860s and became mandatory in the United Kingdom after [[Parliament of the United Kingdom|Parliament]] passed [[Regulation of Railways Act 1889|legislation]] in 1889 following a number of accidents, most notably the [[Armagh rail disaster]]. This required block signalling for all passenger railways, together with [[interlocking]], both of which form the basis of modern signalling practice today. Similar legislation was passed by the United States around the same time.
 
Not all blocks are controlled using fixed signals. On some [[single track (rail)|single track]] railways in the UK, particularly those with low usage, it is common to use [[tokenToken (railway signalling)|token]] systems that rely on the train driver's physical possession of a unique token as authority to occupy the line, normally in addition to fixed signals.
 
=== Entering and leaving a manually controlled block ===
<!-- Absolute block signalling (as is required by UK law since 1889 for all passenger lines in the UK) is operated in a manner designed to ensure two trains may not occupy the same block at once. -->
 
Before allowing a train to enter a block, a signalman must be certain that it is not already occupied. When a train leaves a block, hethey must inform the signalman controlling entry to the block. Even if the signalman receives advice that the previous train has left a block, hethey isare usually required to seek permission from the next signal box to admit the next train. When a train arrives at the end of a block section, before the signalman sends the message that the train has arrived, hethey must be able to see the end-of-train marker on the back of the last vehicle. This ensures that no part of the train has become detached and remains within the section. The end of train marker might be a coloured disc (usually red) by day or a coloured oil or electric lamp (again, usually red). If a train enters the next block before the signalman sees that the disc or lamp is missing, hethey asksask the next signal box to stop the train and investigate.
 
=== Permissive and absolute blocks ===
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{{Main|Automatic block signaling}}
 
The invention of train detection systems such as [[track circuits]] allowed the replacement of manual block systems such as absolute block with automatic block signalling. Under automatic block signalling, signals indicate whether or not a train may enter a block based on automatic train detection indicating whether a block is clear. The signals may also be controlled by a signalman, so that they only provide a ''proceed'' indication if the signalman sets the signal accordingly and the block is clear.
 
=== Fixed block ===
[[File:TTC signal blocks captioned.JPG|thumb|right|Short signal blocks on the [[Toronto Transit Commission]] subway system. A train (not visible) has just passed the most distant, leftmost signal, and the two most distant signals are red ([[Toronto subway and RT signals|''stop and stay'' aspect]]). The next closest signal is yellow (''proceed with caution''), and the nearest signal shows green (''proceed'').]]
 
Most blocks are "fixed", i.e. they include the section of track between two fixed points. On timetable, train order, and [[tokenToken (railway signalling)|token]]-based systems, blocks usually start and end at selected stations. On signalling-based systems, blocks start and end at signals.
 
The lengths of blocks are designed to allow trains to operate as frequently as necessary. A lightly used line might have blocks many [[kilometres]] long, but a busy commuter line might have blocks a few hundred metres long.
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When calculating the size of the blocks, and therefore the spacing between the signals, the following have to be taken into account:
* Line speed (the maximum permitted speed over the line-section)
* Train speed (the maximum speed of different types of traffic)
* Gradient (to compensate for longer or shorter braking distances)
* The braking characteristics of trains (different types of train, e.g., freight, high-speed passenger, have different inertial figures)
* Sighting (how far ahead a driver can see a signal)
* Reaction time (of the driver)
 
Historically, some lines operated so that certain large or high speed trains were signalled under different rules and only given the right of way if two blocks in front of the train were clear.
 
=== Moving block ===<!-- This section is linked from [[Docklands Light Railway]] -->
<!-- [[Docklands Light Railway]] links here. -->
{{Main|Moving block}}
 
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{{Main|Track circuit}}
 
The most common way to determine whether a section of line is occupied is by use of a [[track circuit]]. The rails at either end of each section are electrically isolated from the next section, and an electricalelectric current is fed to both running rails at one end. A [[relay]] at the other end is connected to both rails. When the section is unoccupied, the relay coil completes an electrical circuit, and is energized. However, when a train enters the section, it short-circuits the current in the rails, and the relay is de-energized. This method does not explicitly need to check that the entire train has left the section. If part of the train remains in the section, the track circuit detects that part.
 
This type of circuit detects the absence of trains, both for setting the signal indication and for providing various interlocking functions—for example, preventing points from being moved while a train is approaching them. Electrical circuits also ''prove'' that points are locked in the appropriate position before the signal protecting that route can be cleared. UK trains and staff working in track circuit block areas carry [[Track circuit#Track-circuit operating clips|track circuit operating clips]] (TCOC) so that, in the event of something fouling an adjacent running-line, the track circuit can be short-circuited. This places the signal protecting that line to 'danger' to stop an approaching train before the signaller can be alerted.<ref>{{cite web| title = Rulebook Master: Module M1 Section 3.1 "Dealing with a train accident or evacuation - Providing emergency protection"| url = https://www.rssb.co.uk/rgs/rulebooks/GERM8000-master-module%20Iss%201.pdf| publisher = Network Rail| access-date = 2017-02-12| archive-date = August 10, 2018| archive-url = https://web.archive.org/web/20180810143338/https://www.rssb.co.uk/rgs/rulebooks/GERM8000-master-module%20Iss%201.pdf| url-status = dead}}</ref>
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=== Route signalling and speed signalling ===
''Route signalling'' and ''speed signalling'' are two different ways of notifying trains about junctions.
Most{{Citation needed|date=March 2023}} railway systems use what is known as ''speed signalling''. However, signalling in Britain and in countries influenced by British practice generally conforms to the principle of ''route signalling''.
 
Under '''route signalling''', a driver is informed which route the train will take beyond each signal (unless only one route is possible). This is achieved by a ''route indicator'' attached to the signal. The driver uses histheir route knowledge, reinforced by speed restriction signs fixed at the lineside, to drive the train at the correct speed for the route to be taken. This method has the disadvantage that the driver may be unfamiliar with the required speed over a routejunction onto which hethey hashave been diverted due to some emergency condition. Several accidents have been caused by this alone.<ref name=Rolt>{{cite book|last=Rolt|first=L. T. C.|title=Red for Danger: The Classic History of British Railway Disasters|edition=2nd|year=2009|orig-year=1966|publisher=The History Press|isbn=978-0-7524-5106-0}}</ref> For this reason, in the UK, where all lines are route signalled, drivers are only allowed to drive on routes that they have been trained on and must regularly travel over the lesser used diversionary routes to keep their route knowledge up to date.▼
Note: Generally both Route and Speed signalling follow the exact same rules on straight sections of track without junctions, the differences between the two system arise when there are junctions involved, as the two systems have different methods of notifying trains about junctions.
 
Many route signalling systems use ''approach control'' (see below) to inform a driver of an upcoming change of route.
▲Under '''route signalling''', a driver is informed which route the train will take beyond each signal (unless only one route is possible). This is achieved by a ''route indicator'' attached to the signal. The driver uses his route knowledge, reinforced by speed restriction signs fixed at the lineside, to drive the train at the correct speed for the route to be taken. This method has the disadvantage that the driver may be unfamiliar with a route onto which he has been diverted due to some emergency condition. Several accidents have been caused by this alone.<ref name=Rolt>{{cite book|last=Rolt|first=L. T. C.|title=Red for Danger: The Classic History of British Railway Disasters|edition=2nd|year=2009|orig-year=1966|publisher=The History Press|isbn=978-0-7524-5106-0}}</ref> For this reason, in the UK drivers are only allowed to drive on routes that they have been trained on and must regularly travel over the lesser used diversionary routes to keep their route knowledge up to date.
 
Under '''speed signalling''', the signal aspect informs the driver at what speed hethey may proceed over a junction, but not necessarily the route the train will take. Speed signalling requires a far greater range of signal aspects than route signalling, but less dependence is placed on drivers' route knowledge, although the need for drivers to learn the route is not eliminated as speed signalling does not usually inform drivers of speed limit changes outside junctions. Usually speed limit signs are used in addition to speed signals, with the driver following whichever shows the lower speed.
 
Many systems have come to use elements of both systems to give drivers as much information as possible. This can mean that speed signalling systems may use route indications in conjunction with speed aspects to better inform drivers of their route; for example, route indications may be used at major stations to indicate to arriving trains to which platform they are routed. Likewise, some route signalling systems indicate approach speed using theatre displays so that drivers know what speed they must travel.
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On the 4-aspect system, if the route through the junction is clear the junction signal will display a single ''steady yellow'' aspect together with an illuminated junction indicator showing the selected route.<ref name=flash>{{cite web
| title = Online Rulebook: Signals, Handsignals, Indicators and SignsHandbook RS521 Section 2.5 "Flashing yellow aspects"
| url = http://www.rssb.co.uk/rgs/rulebooks/GERM8000-master-module%20Iss%202.pdf
| publisher = RSSB
| access-date = 18 August 2019
| archive-date = December 5, 2016
| archive-url = https://web.archive.org/web/20161205185841/http://www.rssb.co.uk/rgs/rulebooks/GERM8000-master-module%20Iss%202.pdf
| url-status = dead
}}</ref>
 
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=== Speed-controlled approach ===
[[File:Sydney Trains Trip Dropping Example.gif|thumb|616x616px|Original 1932 operation of Sydney's speed controlled trips. Note: The signal that allows the trains to proceed into the platform is a "call on" signal, andconsisting conflictingof sourcesa saydouble thatred thesignal smallwith lowera lightsmaller wasgreen eitherlight white, yellow or greenunderneath.]]
Some systems in the world use mechanical speed control systems in conjunction with signalling to ensure the speed of a train is limited to a specific value, in order to ensure the train is travelling at a speed in which it is able to stop before an obstruction. These systems most often use mechanical train stop devices (a small arm coming up from the rails that will apply the brakes of a train when run over) to "trip" the brakes of a train that is travelingtravelling too fast. Normally, once a train reaches a certain point on the tracks, it sets off a timer, when the timer runs out the train stop arm will lower, allowing a train to go past uninterrupted. The timing is designed so that if the train is travelingtravelling at the intended speed (or slower) then the train will be able to continue without issue, but if the train is travelingtravelling too fast, then the Train Stop will trip the train and bring it to a halt. This system can be used to ensure a train is travelingtravelling at a certain speed, which allows designers to be confident that shorter signal overlaps will be sufficient, and thus employment of this system can help to greatly improve capacity of a railway line.
 
The system is most often used on approach to dead end junctions to stop trains from crashing into the buffers at the end, as has happened in places such as [[Moorgate tube crash|Moorgate]]. It is also used on high traffic lines to allow for higher capacity, such as the [[City Circle railway line|City Circle Railway]] in Sydney, where it was used on the western half from 1932 to allow 42 trains per hour to traverse the line in each direction, each station would have multiple train stops along the length of the platforms that would progressively lower to ensure an arriving train would not crash into the departing train, less than 100 meters ahead. This system was modified in the early 1990s, so that an arriving train would not be able to enter the platform until the previous train had departed, however the trips continue to be used to overcome the signal overlap normally required.
 
These systems are often used in conjunction with signalling''progressive aspectsspeed that have maximum speedssignalling'' (even in route signallingsee systemsbelow).
 
=== Progressive speed signalling ===
''Progressive speed signalling'' refers to systems that impose speed restrictions on cautionary aspects. On systems that do not have progressive speed signalling, aspects warning of an upcoming red signal do not force the driver to take any action; it is up to their own judgment when to start slowing down in preparation to stop at the red signal. With progressive speed signalling, each cautionary aspect before a red signal imposes a successively lower speed limit on the driver.<ref>{{cite book |last1=Pachl |first1=Jörn |title=Railway Signalling Principles |date=October 2021 |pages=23–25 |edition=2.00 |url=http://www.joernpachl.de/eBook%20RSP.pdf |access-date=27 June 2023}}</ref> It should not be confused with speed signalling as used at junctions; progressive speed signalling can be used in conjunction with route signalling.
 
== Safety systems ==
{{Main|Train protection system}}
 
A train driver failing to respond to a signal's indication can be catastrophic. As a result, various auxiliary safety systems have been devised. Any such system requires installation of some degree of trainbornetrain borne and wayside equipment. Some systems only intervene in the event of a [[signal passed at danger|signal being passed at danger]] (SPAD). Others include audible and/or visual indications inside the driver's cab to supplement the linesideline side signals. Automatic brake application occurs if the driver should fail to acknowledge a warning. The most advanced train control systems have no driver at all relying on computers to drive the system entirely such as Skytrain in Vancouver, Canada and the metro system in Doha, Qatar.
 
In-cab safety systems are of great benefit during [[fog]], when poor visibility would otherwise require that restrictive measures be put in place. Safety systems are also important in urban rail where it is impossible to see around corners in subway and metro tunnels. On-board and wayside computers can track trains around tight corners at higher speeds ensuring safety.
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== Operating rules ==
{{anchor|Operating rules}}
Operating rules, policies and procedures are used by railroads to enhance safety. Specific operating rules mayoften differ from country to country, and there may even be differences between separate railroads within the same country.
 
=== Argentina ===
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{{Main|UK railway signalling}}
The operating rulebook for the United Kingdom is called the "GE/RT8000 Rule Book",<ref>{{Cite web|url =http://www.rgsonline.co.uk/Rule_Book/Forms/Live_Documents.aspx|title =The Rule Book|url-status =dead|archive-url =https://web.archive.org/web/20081214144157/http://www.rgsonline.co.uk/Rule_Book/Forms/Live_Documents.aspx|archive-date =2008-12-14 |publisher=[[Rail Safety and Standards Board|RSSB]] |website=rgsonline.co.uk}}</ref> more commonly known simply as "The Rule Book" by railway employees. It is controlled by the [[Rail Safety and Standards Board]] (RSSB), which is independent from Network Rail or any other [[train operating company]] or [[freight operating company]]. Most [[heritage railway]]s operate to a simplified variant of a [[British Railways]] rule book.
 
=== Finland ===
{{Main|Finnish railway signalling}}
The signalling system used on the [[rail transport in Finland|railway network in Finland]] comprises color-light signals and fixed signs. It is used together with the [[EBICAB]] 900, an automatic train control system better known as JKV, {{langx|fi|junakulunvalvonta}}.
 
=== Italy ===
{{Main|Italian railway signalling}}
In [[Rail transport in Italy|Italy]], railway signalling is described in a particular instruction called [http://site.rfi.it/quadroriferimento/files/RS.pdf ''Regolamento Segnali''] {{Webarchive|url=https://web.archive.org/web/20141207182032/http://site.rfi.it/quadroriferimento/files/RS.pdf |date=December 7, 2014 }} (''Signal Regulation'').
 
=== India ===
The Indian operating rules, called "Thethe "General Rules", are common for all zonal railways of [[Indian Railways]] and can be amended only by the Railway Board. Subsidiary rules are added to the General Rules by zonal railways, which do not infringe theupon general rulesthem. Corrections are brought about from time to time through correction slips.<ref>{{cite web |title=MinistryGeneral ofRules, Railways (Railway Board)1976 |url=https://www.indianrailways.gov.in/railwayboard/view_section.jsp?lang=0&id=0,1,304,366,389,393,1280 |date=29 March 2019 |website=www.indianrailways.gov.in |publisher=IndianMinistry of Railways (Railway Board) |access-date=13 December 2020}}</ref>
 
=== Japan ===
{{Main|Japanese railway signals}}
Japanese railway signalling was initially based on the [[UKBritish railway signalling|British practice, and Japanese railway signalling]] continues to be based on the UK route signalling system for junctions. However, as signalling has advanced to meet the requirements of the modern railway network (and as a result of US influence)system, theprogressive Japanesespeed signalling system is aused mixtureoutside of British route signalling and American speed signallingjunctions.
 
=== Hong Kong ===
Hong Kong Railway signalling originated from the British railway signalling principle,<ref>{{cite web | url=https://www.railsigns.uk/overseas/hongkong1.html | title=Hong Kong }}</ref> and continues to be based on the principles to develop rulebooks and procedures under the operating organisation MTR.
 
== See also ==
{{div col|colwidth=23em}}
* [[Communication Based Train Control]] Signal(signalling Systemsystem)
* [[Gantry (transport)#Railway use|Gantry]]
*[[Institution of Railway Signal Engineers]]
*[[Railroad chronometer]]
*[[Rail sabotage]]
*[[Railroad switch]]
* [[Railway semaphore signal]]
* [[Railway slide fence]]
* [[Signalling block system]]
* [[Signalling control]]
* [[Train speed optimization]]▼
*[[Toronto subway and RT signals]]
▲*[[Train speed optimization]]
* [[Wrong-side failure]]
* [[Norwegian railway signaling]]
{{div col end}}
 
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== General references ==
* {{cite web |last=Brian |first=Frank W. |date=May 1, 2006 |title=Railroad's Traffic Control Systems |url=http://www.trains.com/trn/default.aspx?c=a&id=192 |url-status=dead |website=[[Trains (magazine)|Trains]] |publisher=Kalmbach Publishing Co. |archive-url=https://web.archive.org/web/20071017151346/http://www.trains.com/trn/default.aspx?c=a&id=192 |archive-date=October 17, 2007}}
* {{cite web |last=Colburn |first=Robert |date=October 14, 2013 |title=A History of Railroad Signals |url=http://theinstitute.ieee.org/technology-focus/technology-history/a-history-of-railroad-signals |url-status=dead |website=theinstitute.ieee.org[[IEEE]] |publisher=[[Institute of Electrical and Electronics Engineers]] |archive-url=https://web.archive.org/web/20131022153429/http://theinstitute.ieee.org/technology-focus/technology-history/a-history-of-railroad-signals |archive-date=October 22, 2013}}
* {{cite book |title=General Code of Operating Rules |url=http://fwwr.net/assets/gcor-effective-2015-04-01.pdf |edition=Seventh |workpublisher=GCOR Committee |date=April 1, 2015|archive-url=https://web.archive.org/web/20180619012704/http://fwwr.net/assets/gcor-effective-2015-04-01.pdf|archive-date=19 June 2018}}
* {{cite web |last1=Director of S&T Engineering, West Midlands Project Group [|title=Mechanical Interlocking |url=https://dickthesignals.co.uk/onewebmedia/22%20mechanical%20interlocking.pdf "Mechanical|website=dickthesignals.co.uk Interlocking|publisher=British Rail Board |access-date=26 November 2023 |archive-url=https://web."]archive.org/web/20230418004712/https://dickthesignals.co.uk/onewebmedia/22%20mechanical%20interlocking.pdf |archive-date=18 April 2023 |date=1991}}
 
== External links ==
{{commons cat}}
* [http://www.thesignalpage.nl The Signal Page (TSP) – railway signalling world wide (Dutch)], [http://www.thesignalpage.nl/en/data.php (English)]
* [http://www.railserve.com/Equipment/Signals/ RailServe.com Signals & Communications]
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