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 start, or instructions on what speed the train may go. __TOC__

Aspect and indication

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 over 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. are dark signal head), 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 may be made between absolute signals, which can display a "Stop" indication, and permissive signals, which cannot. Interlocking signals are absolute, while block and distant signals are usually permissive.

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.

see also Token (railway signalling)

Signal mounting

Line side 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 mast which displays the 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 mast 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 mast. The left hand signal then controls the left hand track, and the right signal the right hand track. For mutiple 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.

In some situations there is insufficient room for a mast or gantry, and dwarf signals are used. These are smaller and hold the signal head at ground level. They are also used in combination with mast or gantry signals to control sidings or branch lines. Subway systems commonly use nothing but dwarf signals due to the restricted space.

For a line side signal, the signal head is the portion of the 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. 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 singal facing each direction at each point. At interlockings, however, all tracks are normally signalled.

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 line side. Cab signals display indications for the next block (or other line side signal), rather than for the block currently occupied by the train.

Semaphore signals

Semaphore signals were the most common form of mechanical signals, although they have been largely superseded. The signal head of a semaphore consists of two parts: A blade or vane or arm which pivots at different angles, and a spectacle holding colored lens which move in front of a lamp in order to provide indications at night. Usually these were combined into a single frame, though there were "somersault" signals in which the blade pivoted in the center, separately from the spectacle. The blade projects horizontally over or toward the track it controls in its most restrictive aspect; other angles indicate less restrictive aspects. Semaphores come in "upper quadrant" and "lower quadrant" forms. In lower quadrant signals the blade pivots down for less restrictive aspects. Usually a lower quadrant signal head displayed two aspects, though it was common in the United States for train order signals to point the blade straight down to indicate "Proceed". Upper quadrant signals, as the name implies, pivot the blade upward. These normally display three aspects.

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, pointed or "fishtail" blades for "distant" signals.

Initially semaphores were controlled through mechanical linkages. At an interlocking the switch stand would hold levers to move both the switch points and the signals. Later electromechanical systems used motors to control move the blades remotely, either manually or automatically. The signals are designed so that if power is lost or a linkage is broken, the blade will move by gravity into the horizonal position. For lower quadrant semaphores this requires counterweights to cause the blade to rise rather than fall.

Color-light signals

The introduction of electric light bulbs made it possible to produce colored light signals which were bright enough to be seen during daylight. Many railroads thus converted to color light signals. These came in two forms. The most prevalent form has separate lights 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 ordinarily not used. The lights may be mounted vertically or in a triangle; usually green is on top. Signals with more than three aspects to display generally have multiple heads to display combinations of colors.

In the United States searchlight signals were also used, although these have become less popular. In these a single permanently illuminated incandescent light bulb is used in each head, and a relay is used to position a colored spectacle in front of the bulb. 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.

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 bulbs, 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 bulbs 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 bulb. Approach lighting leaves the signal dark when a train is not present, thus extending the life of the bulbs.

Position light signals

On the Pennsylvania Railroad a system of signals was introduced in which the lights were all the same color (a fog-penetrating yellow) and the aspect consisted solely of the pattern of illuminated lights. the 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 Haven. A similar system was installed on the Norfolk and Western Railway, though these signals were later converted to a color-position system.

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 modify the basic aspect. 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 CSX was replacing these signals with color light signals.

The Norfolk and Western also used a color position system; in their case, however, the aspects were like those of the position light system except with the center light removed and the remaining lights colored to correspond with the pattern of the lights. Instead of auxiliary lights, they used multiple signal heads to get more aspects.

Control and operation 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 points (also called switch or turnout in the US) are set correctly
which way points are set
that the driver has permission to proceed
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:

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

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, allowing him to start braking. Distant signals allowed greater speeds, since trains no longer needed to be able to stop within sight distance of the signals.

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.

In the United States, signal indications at interlockings multiplied to indicate the speed at which the train was to traverse the interlocked trackage. On some railroads up to four different speed classes were authorized, again allowing greater speeds over specially designed trackage. In Great Britain, route signalling was more typical; drivers were expected to know that a diverging route would require a lessening of speed.

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 electrico-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.

Later developments were electric interlocking and controls instead of mechanical, then software interlocking. A development was that mechanical signals were operated by electric motors that moved the signal arms, and current practice is for mechanical signals to be replaced by colour-light signals.

In North America, signalling systems developed independently on different railroads, and different sections of the same railroad commonly used different systems (for example, minor lines often used more primitive and lower capacity systems). Mergers compounded the issue, particularly on the East Coast, where the most idiosyncratic systems were used. The surviving Class I lines have made some efforts at standardization, but rulebooks still have sections listing different aspects and indications for different territories. Smaller lines have often eschewed signals in favor of track warrants or other lower maintenance systems.

The same lack of standaridization which characterizes American signalling also applies to the different national railroad systems in Europe and on other continents.

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