Railway semaphore signal
This semaphore system involves signals that display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'.
[2] The semaphore was swiftly embraced as a fixed manual signaling system across Britain and North America, surpassing all other types in most applications by 1870.
The British semaphore signal arm consists of two parts: A wooden or metal arm (or "blade") which pivots at different angles, and a spectacle holding coloured lenses which move in front of a lamp in order to provide indications at night.
North American semaphore equivalents faced right, and as generally on the European continent, on double track branches, up and down lines are opposite to those in the UK.
Some signals converted to electric lamps from oil, used a yellow-tinted bulb with the original blue lens to maintain the correct colour or from 1996 on the Western Region of British Railways, a 12-volt 5-watt bulb was fitted but fed at 10.7 volts to produce a brilliance approximating to the colour temperature of an oil wick flame, thus providing a correct red or green aspect during darkness.
On some company's lines, a three-aspect semaphore prevailed and these showed an intermediate 'Caution' aspect and thus bore a three-lensed spectacle.
Following nationalisation in 1948, British Railways standardised on upper-quadrant semaphores and all regions gradually adopted that mode, replacing lower-quadrant signals gradually except that the Western Region stayed with their well-proven and nicely proportioned lower-quadrant semaphores with eventually, a heavy cast iron spectacle bearing circular coloured glasses, replacing the former thinly bordered cast spectacle frame with shaped coloured glasses, the main arms being 4 feet (1.2 m) long and subsidiaries 3 feet (0.91 m) with other subsidiaries 2 feet (0.61 m) long.
The Southern Railway in Great Britain frequently made use of old rail for signal posts.
During the 1870s, all the British railway companies standardised on the use of semaphore signals, which were then invariably all of the lower-quadrant type.
[3] From the 1920s onwards, upper-quadrant semaphores almost totally supplanted lower-quadrant signals in Great Britain, except on former GWR lines.
Their spectacle cases, which are on the opposite side of the spindle on which the signal arm is pivoted, are therefore required to be sufficiently heavy to prevent this happening.
[5] Current British practice mandates that semaphore signals, both upper- and lower-quadrant types, are inclined at an angle of 45 degrees from horizontal to display an "off" indication.
[6] The first British railway semaphore signals had arms that could be worked to three positions, in the lower quadrant.
The vertical indication gradually came to be discontinued as the absolute block system superseded time-interval working.
[8] Red was quickly chosen as the best colour for semaphore arms as it was clearly visible against most backgrounds.
Alternatively, part of a bridge abutment conveniently located behind a signal may be painted white.
The levers were grouped together in a building known as a "signal box" (UK and Australia) or "interlocking tower" (North America).
In the UK multiple signal wires were grouped together, and in station areas were commonly routed beneath platform faces just above rail level.
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.
Stop signals are used to protect junctions, points (North America - 'switches'), level crossings, movable bridges, platforms or block sections.
German home semaphore signals (Hauptformsignale) can display a danger aspect if the upper arm is in a 90 degree position (horizontal), and can display a proceed aspect if the upper arm is in a 45 degree position (diagonal).
Home signals may have an additional lower arm which can display a proceed at reduced speed (40 km/h); however, these are not mandatory.
An outer home generally protects a neutral section of 1⁄4 mile (440 yd; 400 m) clear of any points or sidings.
This can be enforced by instruction or by electrical interlocking, which requires the provision of a track circuit on the approach to the signal.
The Ministry of Transport recommended that the colour of distant signal arms and spectacles be altered from red to yellow in the early 1920s, although this was not universally adopted in the UK until the late 1920s.
Both signals display a light at night, which means that the 'danger' indication appears as red over yellow.
These signals may also take the form of a disc with a horizontal stripe which is rotated 45 degrees to the clear position.
In the UK, semaphore signals are as of 2024[update] still present on the national network's secondary routes, though increased spending on renewals has led to a faster rate of decline in recent years.
