Railway signal 1

Railway signalA signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state ofthe line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly. Typically, asignal might inform the driver of the speed at which the train may safely proceed or it may instruct the driver to stop.

Semaphore signals at Worcester Shrub Hill, United Kingdom

Application and positioning ofsignals

Originally, signals displayed simple stop/proceedindications. As traffic density increased, this proved tobe too limiting and refinements were added. One suchrefinement was the addition of distant signals on theapproach to stop signals. The distant signal gave thedriver warning that he was approaching a signal whichmight require a stop. This allowed for an increase inspeed, since trains no longer needed to be able to stopwithin sighting distance of the stop signal.Under timetable and train order operation, the signalsdid 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 onedirection 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 normallyused for bi-directional working). Signals are generally not provided for controlling movements within sidings or yardareas.

Railway signal 2

Aspects and indications

A British lower-quadrant semaphore stop signalwith subsidiary arm below

Signals have aspects and indications. The aspect is the visualappearance of the signal; the indication is the meaning. In Americanpractice the indications have conventional names, so that for instance"Medium Approach" means "Proceed at not exceeding medium speedprepared to stop at next signal". Different railroads historicallyassigned different meanings to the same aspect, so it is common as aresult of mergers to find that different divisions of a modern railroadmay have different rules governing the interpretation of signal aspects.

A Finnish distant signal at the western approachto Muhos station is displaying Expect Stop. In the

background, express train 81 is pulling awayfrom the station.

It is important to understand that for signals that use colored aspects,the color of each individual light is subsumed in the overall pattern. Inthe United States, for example, it is common to see a "Clear" aspectconsisting of a green light above a red light. The red light in thisinstance does not indicate "Stop"; it is simply a component of a largeraspect. Operating rules normally specify that when there is someimperfection in the display of an aspect (e.g., an extinguished lamp),the indication should be read as the most restrictive indicationconsistent with what is displayed.Signals control motion past the point at which the signal stands andinto the next section of track. They may also convey information aboutthe state of the next signal to be encountered. Signals are sometimessaid to "protect" the points/switches, section of track, etc. that they areahead of. The term "ahead of" can be confusing, so official UKpractice is to use the terms in rear of and in advance of. When a train iswaiting at a signal it is "in rear of" that signal and the danger beingprotected by the signal is "in advance of" the train and signal.

A distinction must be made between absolute signals, which candisplay a "Stop" (or "Stop and Stay") indication, and permissivesignals, 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('controlled') signals are typically absolute, while automatic signals (i.e. those controlled through track occupancyalone, not by a signalman) are usually permissive.

Railway signal 3

Drivers need to be aware of which signals are automatic. In current British practice for example, automatic signalshave a white rectangular plate with a black horizontal line across it. In US practice a permissive signal typically isindicated by the presence of a number plate. Some types of signal display separate permissive and absolute stopaspects.Approach lighting leaves the signal dark (or dimmed) when a train is not present. This may be applied for sightingreasons, or simply to extend the life of the lamp and save the batteries. The first such application was on theAtchison, Topeka and Santa Fe Railway on their two arm, lower quadrant Union Switch & Signal (US&S) Style "B"semaphores at the turn of the 19th to the 20th century.Operating rules generally dictate that a dark signal must be interpreted as displaying its most restrictive aspect(generally "Stop" or "Stop and Proceed").

Signal formsSignals differ both in the manner in which they display aspects and in the manner in which they are mounted withrespect to the track.

Mechanical signals

Mechanical semaphore signals at Kościerzyna inPoland

The oldest forms of signal displayed their different indications bya part of the signal being physically moved. The earliest typescomprised a board that was either turned face-on and fully visibleto the driver, or rotated away so as to be practically invisible.These signals had two or at most three positions.Semaphore signals were patented in the early 1840s by JosephJames Stevens, and soon became the most widely-used form ofmechanical signal, although they are now decreasing in number.The semaphore arm consists of two parts: An arm or blade whichpivots at different angles, and a spectacle holding coloured lenseswhich move in front of a lamp in order to provide indications atnight. Usually these were combined into a single frame, though insome types (e.g. "somersault" signals in which the arm pivoted inthe centre), the arm was separate from the spectacle. The armprojects horizontally in its most restrictive aspect; other anglesindicate less restrictive aspects.

Semaphores come in "lower quadrant" and "upper quadrant"forms. In lower quadrant signals, the arm pivots down for lessrestrictive aspects. Upper quadrant signals, as the name implies,pivot the arm upward. Either type may be capable of showing two or three indications depending on the application.For example, it was common in the United States for train order signals to point the arm straight down to indicate"Proceed."The colour and shape of the arm is commonly varied to show the type of signal and therefore type of indicationdisplayed. A common pattern was to use red, square-ended arms for "stop" signals and yellow "fishtail" arms for"distant" signals. A third

Railway signal 4

A British semaphore signal on the former SouthernRegion of British Railways

type with a pointed end extending outward (in the oppositedirection from the fishtail shape) may indicate "proceed atrestricted speed after stopping" (and indeed, stopping itself is oftenwaived for heavy freight ("tonnage") trains already moving at slowspeed).The first railway semaphore was erected by Charles HuttonGregory on the London and Croydon Railway (later the Brighton)at New Cross, southeast London, in the winter of 1842-1843 onthe newly enlarged layout also accommodating the South EasternRailway. The semaphore was afterwards rapidly adopted as a fixedsignal throughout Britain, superseding all other types in most usesby 1870. Such signals were widely adopted in the U.S. after 1908.

Initially, railway semaphores were mounted on the roof of thecontrolling signal box, but gradually a system of wires and pulleyscontrolled through mechanical linkages was developed to controlthe signals at a distance. Signal boxes became controllers ofinterlockings, and came to be known as interlocking towers orsimply signal towers in the United States, while retaining the name"signal box" in the United Kingdom. The signals protecting thestation itself came to be called home signals, while signals somedistance away giving advance warning came to be called distantsignals.

Mechanical signals may be operated by electric motors or hydraulically. The signals are designed to be fail-safe sothat if power is lost or a linkage is broken, the arm will move by gravity into the horizontal position. For lowerquadrant semaphores, the positioning of the spectacles and their housing on the opposite side of the pivot to the armwill force the signal to return to danger - the arm itself is relatively light compared to these.In the U.S., semaphores were employed as train order signals,[1] with the purpose of indicating to engineers whetherthey should stop to receive a telegraphed order, and also as simply one form of block signalling. Mechanical signalsworldwide are being phased out in favour of colour-light signals or, in some cases, signalling systems that do notrequire lineside signals (e.g. Radio Electronic Token Block or RETB).

Railway signal 5

Colour light signals

Network Rail (UK) two-aspect colour lightrailway signal set at 'danger'

German railway signals showing aspect Hp0(Stop)

The introduction of electric light bulbs made it possible to producecolour light signals which were bright enough to be seen duringdaylight, starting in 1904. These at first were "short range" signals,used in low speed applications. Improvements in signal glassware byDr. Churchill of Cornell University in conjunction with Corning GlassWorks (Corning, New York) increased the daylight viewing range tothat of "medium range" signals. These were quickly adopted by manyU.S. rapid transit and trolley (street, light) railway systems. Ultimately,by 1913, Dr. Churchill developed the "doublet lens" combinationwherein an outer clear Fresnel lens was sealed with inwards facing ribsto an inner glass of the desired color with ribs facing the outer lens.Both inner and outer exposed surfaces were therefore smooth, avoidingdirt accumulation. The inner lens convexity permitted a 35 watt, 10volt lamp filament to be almost surrounded by this system, gatheringmuch more of the light produced than with previous systems. Thisyielded the first true "Long Range" daylight signal with a range of over2500 feet (760 m) under bright sun conditions. The first applicationwas by the Chicago, Milwaukee, St. Paul and Pacific Railroad's use ofthe US&S "Style L" colour light signal on their line through the PacificNorthwest in 1914. These were intentionally fitted from the start withtwo lamps one ahead of the other, the circuitry so arranged that failureof the main lamp caused the lighting the back-up lamp to assure themost restrictive indication always being illuminated. The finalimprovement came in the early 1920s with Corning's "HighTransmission" glass colours increasing this range to 3500 feet(1100 m) under bright sun conditions. The results were that in the U.S.,all semaphore manufacturing had ceased by 1944, having beencompletely replaced with light signals of one type or another.

The signal head is the portion of a colour light signal which displaysthe aspects. To display a larger number of indications, a single signalmight have multiple signal heads. Some systems used a single headcoupled with auxiliary lights to modify the basic aspect.

Colour light signals come in two forms; both were developed in thefirst quarter of the 20th century in the U.S. The most prevalent form isthe multi-unit type, with separate lights and lenses for each colour, inthe manner of a traffic light. Hoods and shields are generally providedto shade the lights from sunlight which could cause false indications;coloured Fresnel lenses are used to focus the beam, though reflectorsare often not used, to prevent false indications from reflected sunlight.The lights may be mounted vertically or in a triangle; usually green ison top and red at the bottom. Signals with more than three aspects todisplay generally have multiple heads to display combinations ofcolours.

Railway signal 6

Mechanism of a searchlight signal made byUnion Switch & Signal, with the lamp andreflector removed to expose the coloured

roundels

Searchlight signals were the most often used signal type in the U.S.until recently, although these have become less popular due tovandalism. In these, a single incandescent light bulb is used in eachhead, and either an A.C. or D.C. relay mechanism is used to move acoloured spectacle (or "roundel") in front of the lamp. In this manner,gravity (fail safe) returns the red roundel into the lamp's optical path. Ineffect, this mechanism is very similar to the colour light signal that isincluded in an electrically operated semaphore signal, except that theomission of the semaphore arm allows the roundels to be miniaturizedand enclosed in a weatherproof housing. An elliptical reflector focusesthe lamp through the roundel a small lens and then a larger single orcompound lensing arrangement is employed.[2] The Hall SignalCompany first produced this type of signal in 1920, having purchased

the patent rights from one Mr. Blake in 1918. US&S bought the Hall Company in 1925 thus naming this signal theStyle "H," with the later compound lens version (introduced in 1932) referred to as the Style "H-2." The GeneralRailway Signal Company's versions were known as the "Type S, SA, SA-1 and SC", the last having a sequentialoperating mechanism that could be used to directly replace motor-operated semaphores without any changes insystem wiring.

The viewing angle for the searchlight beam, though narrow, is enormously powerful. Initial range was in excess ofone mile and that was in 1918. Deflecting cover glasses are used to give a spread of the indication on non tangenttrack as needed. Again, to display more than three aspects, multiple heads are used as is standard U.S. practice, thatof A.H. Rudd's 1915 "Speed Signalling System." Searchlight signals have the disadvantage of having moving partswhich may be deliberately tampered with. From World War II onward, they were widely used in the U.S. for newwork until the last fifteen to twenty years when vandalism began to render them vulnerable to false indications.A variant of the above, a "medium range" single lens signal, is the "Unilens" signal made by Safetran SystemsCorporation. This uses a single-lens system, fed by three or four individual halogen lamps with parabolic reflectorsbehind them. These lamps shine through the same coloured roundels used in searchlight signals into individualfibre-optic elements, which are rolled together as sheets to make a solid cable as it were at the focal point of the lensassembly. This makes it possible to show four different colours (usually red/yellow/green/lunar white) from a singlesignal head, which is not possible for the traditional searchlight mechanism, nor is it necessary. The marginalmedium range aspect of this signal is often somewhat compensated by the use of the illumination of two red lampssimultaneously lit to improve visual range. Current consumption of this arrangement would have been prohibitiveduring the 20th century, when most signals were battery powered.More recently, clusters of LEDs 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. Theseare often arranged so that the same aperture is used for whichever colour light is required, but again only for mediumor short range use as the number per colour of discrete LEDs is much less than found on high signals intended forlong range application.Operating rules generally dictate that a dark signal be interpreted as giving the most restrictive indication it candisplay (generally "stop" or "stop and proceed"). Obviously this greatly impedes traffic until repairs are made.Therefore many colour light systems have circuitry to detect failures in lamps or mechanism, allowing the signal tocompensate for the failure by displaying an aspect which, while more restrictive than that set by the dispatcher orsignalling equipment, still allows traffic to pass; for example, if a green lamp is burned out, but the indication to bedisplayed is "clear", the signal can detect this and display a cautionary aspect using a different lamp or lamps,allowing traffic to proceed at reduced speeds without stopping.

Railway signal 7

In the UK, most filament-type colour light signals are equipped with lamps having two filaments. When the mainfilament fails, the auxiliary filament automatically comes into use. Failure of the main filament is indicated to thetechnician (but not the signalman), who will then arrange for the lamp to be replaced. Failure of both filaments,resulting in a 'dark' signal, is indicated to the signalman, inside the signal box; also, the previous signal may also berestricted to no more than a yellow warning aspect.

Position light signals

PRR position light signal

A position light signal is one where the position of the lights, ratherthan their colour, determines the meaning. The aspect consists solely ofa pattern of illuminated lights, which are all of the same colour(typically amber or white). In many countries, small position lightsignals are used as shunting signals, while the main signals are ofcolour light form. Also, many tramway systems (such as the Metro ofWolverhampton) use position light signals.

On the Pennsylvania Railroad (PRR) as on other railroads, initialefforts were made to replace the semaphore with illumination of theposition of the blade rather than by colour lamps alone. This had theadvantage of eliminating any and all effects of even slight colourblindness by the train crew. Lamps with inverted half toric optic lenses,covered with a light yellow tinted conical cover glass with a frosted tipto avoid phantom indications were displayed in rows of three,corresponding to the positions of a semaphore blade. Multiple signalheads were used at interlockings where four aspects did not suffice. The PRR chose to use their Superintendent ofSignaling, A.H.Rudd's, in-house developed position light signals to both replace the semaphores and their movingparts, also because the intense lemon yellow light provided superior visibility in adverse weather conditions such asrain or fog. The prototype position lights used rows of four lamps in an asymmetric fashion in the style of semaphoreblades, but this was later changed to the symmetric three-lamp system. The first installation of the four 5 volt, 10watt lamp position light signals occurred on the Main Line between Philadelphia and Paoli, in conjunction with the1915 electrification. These first signals differed from the later ones in that the lamps were mounted separately infront of a tombstone-shaped black painted metal backing. There were found issues with wind damage due to therather larger "sail area" of the "tombstone backing." Soon thereafter, the lamps were reduced in number to three perrow, without adversely affecting the long range viewing of the signal aspects and the backing correspondinglyreduced in size and made as a disc. The lamp units and background disc on a mounting system known as a "spider,"were integrated into a single unit.

Railway signal 8

Two-head colour position signal on CSXTmainline at Savage, Maryland. The left head

displays "Stop"; the right head, "Clear".

Colour-position signals

A system combining aspects of the colour and position systems wasdeveloped on the Baltimore and Ohio Railroad (B&O) in 1920 and waspatented by L.F. Loree and F.P. Patenall. In effect, it is a "digital"semaphore. It was also applied to the Chicago and Alton Railroadwhen the latter was under B&O control. The color position lights(CPLs) were first installed as a pilot on the Staten Island Railway inNew York City, a former B&O subsidiary, later turned rapid transitline operated by the Metropolitan Transportation Authority. The B&Osystem used a central round head with pairs of lights mimicking thespeed signalling system of semaphore positions developed by A.H.Rudd and accepted as standard by the American Railway AssociationSignal Section in 1915. It used pairs of large coloured lights (green |,yellow /, red --). with a lunar white \ also being present in someinstallations.

Signal mounting

A gantry of British semaphore signals seen fromthe cab of a steam locomotive

Lineside signals need to be mounted in proximity to the track whichthey control.

Post mounting

When a single track is involved, the signal is normally mounted on apost which displays the arm or signal head at some height above thetrack, in order to allow it to be seen at a distance. The signal isnormally put on the engine driver's side of the track.

Gantry mountingWhen multiple tracks are involved, or where space does not permit post mounting, other forms are found. In doubletrack territory one may find two signals mounted side by side on a bracket which itself is mounted on a post. The lefthand signal then controls the left-hand track, and the right signal the right-hand track. A gantry or signal bridgemay also be used. This consists of a platform extending over the tracks; the signals are mounted on this platformover the tracks they control.

Railway signal 9

Ground mounting

Dwarf signal at Utrecht Centraal, Netherlands

In some situations where there is insufficient room for a post or gantry,signals may be mounted at ground level. Such signals may bephysically smaller (termed dwarf signals). Rapid transit systemscommonly use nothing but dwarf signals due to the restricted space. Inmany systems, dwarf signals are only used to display 'restrictive'aspects such as low speed or shunt aspects, and do not normallyindicate 'running' aspects.

Other

Occasionally, a signal may be mounted to a structure such as aretaining wall, bridge abutment, or overhead electrification support.

FilamentsElectric lamps for railway signals are often fitted with twin filaments, so that if one burns out, the other keeps thesignal lit. A more complicated version of this, such as in the SL35 lamp, a filament changeover relay is fitted inseries with the first filament, where if the first filament burns out, the relay drops and lights the second filament. Thisfilament fail relay also activates an alarm in the signal box.

Lamp provingWhen lamps fail, this can result in aspects that are less restrictive (high speed) that 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 thefailed feather indicator, and prevent the green from showing. It can also display an indication on the signaller's panel.

Control and operation of signalsSignals were originally controlled by levers situated at the signals, and later by levers grouped together andconnected to the signal by wire cables, or pipes supported on rollers (US). Often these levers were placed in a specialbuilding, known as a signal box (UK) or interlocking tower (US), and eventually they were mechanically interlockedto prevent the display of a signal contrary to the alignment of the switch points. Automatic traffic control systemsadded track circuits to detect the presence of trains and alter signal aspects to reflect their presence or absence.

Cab signallingSome locomotives are equipped to display cab signals. These can display signal indications through patterns oflights in the locomotive cab, or in simple systems merely produce an audible sound to warn the driver of a restrictiveaspect. Occasionally, cab signals are used by themselves, but more commonly they are used to supplement signalsplaced 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 movementauthority.

Railway signal 10

Signalling powerUsually, signals and other equipment (such as track circuits and level crossing equipment), are powered from a lowvoltage supply. The specific voltage varies with the country and equipment used. The reason behind this is that thelow voltage allows easy operation from storage batteries and indeed, in some parts of the world (and previously inmany more locations, before the widespread adoption of electricity), batteries are the primary power source, as mainspower may be unavailable at that location. In urban built-up areas, the trend is now to power signal equipmentdirectly from mains power, with batteries only as backup.

References• Armstrong, John (1957). "All About Signals" (Two-article series). Trains Magazine [3], June and July 1957.• Calvert, J.B. "The Origin of the Railway Semaphore." [4] 2007-05-04.• Kichenside, G. and Williams, A., (1998), Two Centuries of Railway Signalling, Oxford Publishing Co., ISBN

0-86093-541-8• Vanns, M.A., (1995), Signalling in the Age of Steam, Ian Allan, ISBN 0-7110-2350-6

Notes[1] Calvert, J.B. "Train Order Signals." (http:/ / www. du. edu/ ~etuttle/ rail/ to. htm) Railways: History, Signalling, Engineering. 2004-08-10.[2] Railway Signal Site. "US&S Searchlight Signal H and H2 Styles." (http:/ / www. trainweb. org/ signals/ usssls_types. htm) Accessed

2011-09-11.[3] http:/ / www. trains. com/ trn/[4] http:/ / www. du. edu/ ~jcalvert/ railway/ railhom. htm#hist

External links• Signal Page (http:/ / www. thesignalpage. nl) - world wide railway signalling info and references

Article Sources and Contributors 11

Article Sources and ContributorsRailway signal  Source: http://en.wikipedia.org/w/index.php?oldid=450783086  Contributors: 2T, 7severn7, AB, Adambro, Ae-a, Amakuru, AmosWolfe, Benet Allen, BesigedB, Biruitorul,Black Kite, Bobblewik, BruceDLimber, Bvi2002, CORNELIUSSEON, Caliban666, Caseyjonz, Cctoide, ChrisJ, Cluth, Dan100, Danny252, Douglas W. Jones, DragonHawk, EdJogg,EngineeringCat, Ephebi, Esrever, G-Man, Gaius Cornelius, GeertW, Goose, Hellbus, Hertzsprung, JackLumber, Jezhotwells, Jmchuff, JonRoma, Joseph Solis in Australia, Jp347, Jswd, Krallja,Lightmouse, Likelife, LorenzoB, Lumos3, Magnus242, Mangoe, Michael Hardy, Mintleaf, Modelbouwmaster, Mwng, N17mikee, Ncsa3809, Nk, Oknazevad, Oldboltonian, Once in a BlueMoon, Oxonhutch, Patrick, Paul Lubliner, Peter Horn, Philip J. Rayment, Picapica, QuantumEngineer, Rackellar, Radagast83, Rafikk, Realkyhick, Redvers, Retro00064, Rhysjones25, RichFarmbrough, Rjd0060, Rjwilmsi, Sam Hocevar, Sansculotte, Sf5xeplus, Shantavira, Shrdlu junction, Signalhead, Skabat169, Slambo, Stephan Leeds, Sturmovik, Suicidalhamster, Tabletop,Tam0031, Tarkan1st, TarzanASG, TenOfAllTrades, TheMadBaron, Tide rolls, Tim Zukas, TimeriderTech, Tpoulton, Train2104, Trap-door, Truthanado, Tve4, Vanished user 03, Wongm,Wychegnome, 134 anonymous edits

Image Sources, Licenses and ContributorsImage:wsh_signals.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Wsh_signals.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: SignalheadImage:Rail-semaphore-signal-Dave-F.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Rail-semaphore-signal-Dave-F.jpg  License: Creative Commons Attribution 2.0  Contributors:Original uploader was AmosWolfe at en.wikipediaImage:Finnish distant signal displaying Expect Stop.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Finnish_distant_signal_displaying_Expect_Stop.jpg  License: Public Domain Contributors: User:TeVeImage:Rail semaphore koscierzyna.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Rail_semaphore_koscierzyna.jpg  License: Creative Commons Attribution-Sharealike 2.0 Contributors: Rafal KonkolewskiImage:Semaphore Signal on British Rail Southern Region Redvers.jpg  Source:http://en.wikipedia.org/w/index.php?title=File:Semaphore_Signal_on_British_Rail_Southern_Region_Redvers.jpg  License: Public Domain  Contributors: Caseyjonz, RedversImage:Two-aspect-signal.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Two-aspect-signal.jpg  License: Public Domain  Contributors: Andrew Bell (en:User:BesigedB)Image:German HP signals.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:German_HP_signals.jpg  License: Public Domain  Contributors: NominativusImage:SearchlightSignalMechanism.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:SearchlightSignalMechanism.JPG  License: Public Domain  Contributors: Douglas W. JonesFile:OVERBROOK_18R-Approach-Medium.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:OVERBROOK_18R-Approach-Medium.jpg  License: Creative CommonsAttribution-Sharealike 3.0  Contributors: SturmovikImage:CSX color position light Savage MD.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CSX_color_position_light_Savage_MD.jpg  License: Creative Commons Attribution 2.5 Contributors: MangoeImage:SR signals from cab of standard five.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:SR_signals_from_cab_of_standard_five.jpg  License: Copyrighted free use Contributors: Caseyjonz, Duncharris, L.m.k, Mattbuck, The NavigatorsImage:Railway signal at Utrecht Centraal.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Railway_signal_at_Utrecht_Centraal.jpg  License: Creative CommonsAttribution-Sharealike 2.5  Contributors: G. Wirken

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