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A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train drivers/engineers. 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.

Semaphore railway signals at the National Railway Museum, York

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.
• 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 one direction only, with all signals facing the same direction on either line. Where 'bi-directional' signalling is installed, signals face in both direction on both lines (sometimes known as 'reversible working' where lines not normally used for bi-directional working). Signals are generally not provided for controlling movements within sidings or yard areas.

A British lower-quadrant semaphore stop signal with subsidiary arm below.

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 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.

A Finnish distant signal at the western approach to Muhos station is displaying Expect Stop. In the background, express train 81 is pulling away from 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. In the United States, for example, it is common to see a "Clear" aspect consisting of a green light above a red light. The red light in this instance does not indicate "Stop"; it is simply a component of a larger aspect. Operating rules normally specify that when there is some imperfection in the display of an aspect (e.g., an extinguished lamp), the indication should be read as the most restrictive indication consistent with what is displayed.

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/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.

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 ('controlled') signals are typically absolute, while automatic signals (i.e. those controlled through track occupancy alone, not by a signalman) are usually permissive.^{[verification needed]}

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.

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 semaphore signals at Kościerzyna in Poland

A British semaphore signal on the former Southern Region of British Railways

The oldest forms of signal displayed 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.

Semaphore signals were patented in the 1840s by Joseph James Stevens, and soon became the most widely-used form of mechanical signal, although they are now decreasing in number. The semaphore arm consists of two parts: A blade or vane or arm which pivots at different angles, and a spectacle holding colored lenses which move in front of a lamp in order to provide indications at night. Usually these were combined into a single frame, though in some types (e.g. "somersault" signals in which the blade pivoted in the center), the arm was separate from the spectacle. The blade projects horizontally in its most restrictive aspect; other angles indicate less restrictive aspects. Semaphores come in "lower quadrant" and "upper quadrant" forms. In lower quadrant signals, the blade pivots down for less restrictive aspects. Upper quadrant signals, as the name implies, pivot the blade 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 blade straight down to indicate "Proceed". In India, the mechanical signals are referred as "Die" when the blade is 90 degrees and "Do" when the blade is angled lower or upper.

The color and shape of the blade is commonly varied to show the type of signal and therefore type of indication displayed. A common pattern was to use red, square-ended blades for "home" signals and yellow "fishtail" blades for "distant" signals. A third type with a pointed end extending outward (in the opposite direction from the fishtail shape) may indicate "proceed at restricted speed after stopping" (and indeed, stopping itself is often waived for heavy freight ("tonnage") trains already moving at slow speed.

Initially, semaphores were controlled through mechanical linkages. At an interlocking, the signal box (UK) or tower (US) originally held levers to move both the switches (points) and the signals. Mechanical signals may be operated by electric motors or hydraulically. The signals are designed to be fail-safe so that if power is lost or a linkage is broken, the blade will move by gravity into the horizontal position. For lower quadrant semaphores this requires special counterweights to cause the blade to rise rather than fall; this is one of the reasons for the widespread switch to upper quadrant signals.

Mechanical signals are being phased out in favour of colour-light signals or, in some cases, signalling systems that do not require lineside signals (e.g. RETB).

Network Rail two-aspect colour light railway signal set at danger

The introduction of electric light bulbs made it possible to produce color light signals which were bright enough to be seen during daylight. Many railroads thus converted to color light signals.

The signal head is the portion of a color 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.

Color light signals come in two forms. The most prevalent form is the multi-unit type, with separate lights and lenses for each color, in the manner of a traffic light. Hoods and shields are generally provided to shade the lights from sunlight which could cause false indications; colored Fresnel lenses are used to focus the beam, though reflectors are often not used in order to prevent false indications from reflected sunlight. The lights may be mounted vertically or in a triangle; usually green is on top and red at the bottom. Signals with more than three aspects to display generally have multiple heads to display combinations of colors.

Searchlight signals were also used, although these have become less popular. In these, a single incandescent light bulb is used in each head, and a solenoid is used to position a colored spectacle (or 'roundel') in front of the lamp. These use both lenses and reflectors (since the color cannot be washed out by the sun) and often have to be carefully sighted in order for the light to be seen properly. Again, to display more than three aspects, multiple heads are used. Searchlight signals have the disadvantage of having moving parts in what can be a hostile location for mechanical equipment and thus need regular maintenance. Examples of searchlight signals in the UK can still be found on the Colchester to Clacton Line.

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 colored 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 lamps, reflectors and lenses. They have a more even color 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.

Many color light systems have circuitry to detect failures in lamps or mechanism, allowing the signal to compensate for the failure by changing to an aspect that is less restrictive than that indicated with the dark lamp. Approach lighting leaves the signal dark when a train is not present. This may be applied for sighting reasons, or simply to extend the life of the lamp.

In the UK, most filament-type colour light signals are equipped with lamps having two filaments. When the main filament fails, the auxiliary filament automatically comes into use. Failure of the main filament is indicated to the technician (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.

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

On the Pennsylvania Railroad, lights were displayed in rows of three, corresponding to the positions of an upper quadrant semaphore blade. Multiple signal heads were used at interlockings where three aspects did not suffice. The Pennsylvania Railroad chose to use position lights to both replace the semaphores and their moving parts as well as because the intense amber light provided superior visibility in adverse weather conditions such as rain or fog. The prototype position lights used rows of 4 lamps in an asymmetric fashion in the style of semaphore blades, but this was later changed to the symmetric 3-row system. The first installation of 3-lamp semaphores occurred on the Main Line between Philadelphia and Paoli in concurrence with the 1915 electrification. These first signals differed from the later ones in that the lamps were mounted separately in front of a tombstone shaped black painted metal backing. Later the lamps and backing were integrated into a single unit.

The Norfolk and Western also adopted PRR type amber position lights as the PRR had a 33% in the M&W at the time. Furthermore, the Long Island Rail Road adopted position lights after it was bought outright by the PRR. After the Penn Central merger, the former all-amber position lights were modified with twin red lenses in the upper horizontal position for enhanced recognizability of Stop signals at interlockings. The N&W also modified its all-amber position lights to include colour in the 1950's and Amtrak fully colourized its inherited position lights (replacing all the amber) starting in the 1980's.

Two-head color position signal on CSXT mainline at Savage, Maryland. The left head displays "Stop", the right head, "Clear".

A system combining aspects of the color and position systems was developed on the Baltimore and Ohio Railroad in the 1920s and was also applied to the Chicago and Alton Railroad when the latter was under B&O control. The CPLs were first installed as a pilot on the Staten Island Railroad in New York City, a former B&O subsidiary later turned rapid transit line operated by the Metropolitan Transportation Authority. The B&O system used a central round head with pairs of lights mimicking the traditional semaphore positions using pairs of large coloured lights (green |, yellow /, red --) with a lunar white \ also being present sometimes. The main head was surrounded by up to 6 so-called "orbitals" at the 12, 2, 4, 6, 8 and 10 o'clock positions. The function of the main head was block occupancy information with green representing 2 or more clear blocks, yellow 1 clear block and red/lunar white representing no clear blocks. The orbitals would then serve to provide speed information, 12 o'clock being full (authorized) speed, 6 being Medium speed (Limited speed if flashing), 10 being Full to Medium (Limited if flashing), 2 being Full to Slow, 8 being Medium to Medium, 4 being Medium to Slow and no lit orbitals being Slow to Slow.

The B&O CPL system was, and continues to be, the most theoretically sound signaling system in North America. It is the only system of signal aspects used in North America which only displays the colour red for situations involving an obstructed block or interlocking. Also, it is the only system to use the same aspects on high signals as it does on dwarf signals. Despite its advantages in clarity and viability, due to higher maintenance and construction costs it was not adopted by other railroads, and in the 1990s and 2000s CSX was gradually replacing these signals with color light signals, though as of 2006, clusters of them remained, especially on secondary main lines. When the Staten Island Railroad was re-signaled in 2005 the MTA decided to keep and upgrade the CPL system.

The Norfolk and Western as well as Amtrak both used a system which altered former all-amber position lights to ones with coloured lenses for visibility purposes. These should not be referred to or mistaken with B&O Colour Position Lights. On Amtrak they are officially called Position Colour Light although colourized position light would also be accurate.

A gantry of British semaphore signals seen from the cab of a steam locomotive.

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

When a single track is involved, the signal is normally mounted on a post 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 put on the engineer's or driver's side of the track.

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. For multiple tracks a gantry or signal bridge is also used. This consists of a platform extending over the tracks; the signals are mounted on this platform over the tracks they control.

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 be physically smaller (termed dwarf signals). Rapid transit systems commonly use nothing but dwarf signals due to the restricted space.

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

Originally, signals displayed simple stop/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/engineer warning that he was approaching a signal at which he might have to stop. This allowed for an increase in speed, since trains no longer needed to be able to stop within sighting distance of the stop signal.

Under timetable and train order 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 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 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 circuits to detect the presence of trains and alter signal aspects to reflect their presence or absence.

Main article: Cab signalling

Some locomotives are equipped to display cab signals. These display signal indications through patterns of lights in the locomotive cab. On some lines, cab signals are used by themselves, but more commonly they are used to supplement signals placed at lineside. Cab signals display indications for the next block (or other lineside signal), rather than for the block currently occupied by the train. 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.

The examples and perspective in this article or section may not represent a worldwide view of the subject. Please improve this article or discuss the issue on the talk page.

Most signals require electrical power, or at least detection circuitry to interface with modern signal boxes. This equipment normally operates at 110V and is sourced from local lineside location cabinets known as FSPs (Function Supply Points). These contain transformers to step down the voltage from the main distribution voltage of 650V single phase, that is supplied from the PSP (Principal Supply Point). PSPs contains transformers and switchgear associated with providing the 650V distribution system and can also contain generator and UPS backup systems. There are also some systems that operate at 400V three phase and some smaller locations at 230V single phase.^{[citation needed]}

• 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

• List of rail diagrams
• Railway semaphores
• Train protection system
• Train speed optimization

Wikimedia Commons has media related to:

Railway signals

• The Signal Page (TSP) is a Dutch website about railway signalling, world wide.
• John Hinson's site 'The Signal Box'
• Railway Signs and Signals of Great Britain
• Danish Signaling, an introduction
• Clive Feather's Junction Signalling
• Signalling Record Society
• French Signalling
• The European Railway Signalling Server
• The Institution of Railway Signal Engineers