Railway signal

A signal is a mechanical or electrical device that indicates to train drivers or engineers information about the state of the line ahead, and therefore whether he or she must stop or may proceed, or instructions on what speed the train may go.

Application and positioning of signals

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 (with block signalling).
on the approach to points/switches.
in advance of other signals.
on the approach to level crossing.
ahead of platforms or other places that trains are likely to be stopped.
at train order stations.

Not all tracks may be signalled at a given location. On double track lines it was not uncommon to find that each track was signalled in one direction only, so that there would be only one signal facing each direction at each point. At interlockings, however, all tracks are normally signalled.

Aspects and indications

A British lower-quadrant semaphore home signal with calling-on 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.

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]}

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

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 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).

Color light signals

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 to 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.

Position light signals

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 white or yellow). 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. Amtrak's Northeast Corridor still uses this type of signals south of New Rochelle, N.Y., except that at most locations the lamps have been replaced with color lights so that they resemble the color-position signals common on the Norfolk and Western Railway.

In many countries, small position light signals are used as shunting signals, while the main signals are of color light form.

File:CSX color position light Savage MD.jpg

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

Color-position signals

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. This system uses a main signal head which has pairs of colored lights. Two vertical lights are green, two yellow are diagonal, two horizontal are red, and two reverse diagonal are lunar white. Additional lights are arrayed above and/or below the main head to indicate speed (unrestricted, "medium", "slow"). This system, as with the position light system, allowed reading the aspect correctly if one of the bulbs failed. 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.

The Norfolk and Western also used a color position system. In these signals, the aspects were like those of the position light system without the center light, and the remaining lights colored to correspond with the pattern of the lights. Instead of auxiliary lights, multiple signal heads were used to obtain additional aspects.

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 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.

Gantry mounting

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.

Ground mounting

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.

Control and operation of signals

Originally signals were manually operated to display simple stop/proceed directions. As traffic density increased, this proved to be too limiting, and refinements were added. One such refinement was the addition of distant signals in advance of absolute (home) signals. The distant signal gave the driver or engineer warning that he was approaching a signal at which he would have to stop. This allowed greater speeds, since trains no longer needed to be able to stop within sighting distance of the home 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 in the signal box to prevent the display of a signal contrary to the alignment of the switch points. Mechanical interlockings were gradually replaced by electro-mechanical and fully electrically operated systems. Automatic traffic control systems added block circuits to detect the presence of trains and alter signal aspects to reflect their presence or absence. The most modern systems are computer controlled, though there are often redundant interlockings to prevent the software from putting the signals and trackage into an unsafe state.

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.

References

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

External links