Railway signal
In current British practice for example, automatic signals have a white rectangular plate with a black horizontal line across it.
[4] Some types of signal display separate permissive and absolute stop aspects.
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.
Gregory's installation was inspected and approved for the Board of Trade by Major-General Charles Pasley.
The invention of the electric light, which could be made brighter than oil lamps and hence visible both by night and day, resulted in the development of position light signals and colour-light signals at the beginning of the 20th century,[11] which gradually displaced semaphores.
[13] Mechanical signals may be operated manually, connected to a lever in a signal-box, 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 arm will move by gravity into the horizontal position.
Some systems used a single head coupled with auxiliary lights to modify the basic aspect.
Hoods and shields are generally provided to shade the lights from sunlight which could cause false indications.
[citation needed] In these, a single incandescent light bulb is used in each head, and either an A.C. or D.C. relay mechanism is used to move a coloured spectacle (or "roundel") in front of the lamp.
In this manner, gravity (fail safe) returns the red roundel into the lamp's optical path.
In effect, this mechanism is very similar to the colour light signal that is included in an electrically operated semaphore signal, except that the omission of the semaphore arm allows the roundels to be miniaturized and enclosed in a weatherproof housing.
[15] Widely used in the U.S. from World War II onward, searchlight signals have the disadvantage of having moving parts which may be deliberately tampered with.
This had led to them becoming less common during the last fifteen to twenty years when vandalism began to render them vulnerable to false indications.
However, in some other countries, such as on the Italian railways (FS) as from the Regolamento Segnali, they are still the standard colour light signal albeit with new installations being as outlined below.
More recently, clusters of LEDs have started to be used in place of the incandescent lamps, reflectors and lenses.
An additional pair, colored "lunar white", may be added on the other diagonal for restricting indications.
Colour position lights (CPLs) were first installed as a pilot on the Staten Island Railway in New York City, at the time a B&O subsidiary; they were also applied to the Chicago and Alton Railroad when the latter was under B&O control, as well as on the B&O itself.
When multiple tracks are involved, or where space does not permit post mounting, other forms are found.
In some situations or places, such as in tunnels, where there is insufficient room for a post or gantry, signals may be mounted at ground level.
Rapid transit systems commonly use only dwarf signals due to restricted space.
Occasionally, a signal may be mounted to a structure such as a retaining wall, bridge abutment, or overhead electrification support.
These can display signal indications through patterns of lights in the locomotive cab, or in simple systems merely produce an audible sound to warn the driver of a restrictive aspect.
The reason behind this is that the low voltage allows easy operation from storage batteries and indeed, in some parts of the world (and previously in many more locations, before the widespread adoption of electricity), batteries are the primary power source, as mains power may be unavailable at that location.
