One of the earliest forms of fixed railway signal is the semaphore. These signals display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'. Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely-used form of mechanical signal. Designs have altered over the intervening years, and colour light signals have replaced semaphore signals in some countries, but in others they remain in use.
The appropriately coloured lens, depending on the arm's position, is illuminated from behind by either an oil lamp or an incandescent lamp run at a low voltage (white LED clusters have also been trialled for this purpose). Where a green light was required, a blue lens would usually be used. When combined with the mainly yellow-emitting flame of an oil lamp, this produced a green colour. Later signals using electric lamps used green lenses. Some signals converted to electric lamps from oil used a yellow-tinted bulb with the original blue lens to maintain the correct colour.
Materials that were commonly used to make signal posts for semaphore signals included timber, lattice steel, tubular steel and concrete. The Southern Railway in Great Britain frequently made use of old rail for signal posts.
Semaphores come in lower quadrant and upper quadrant forms. In a lower quadrant signal, the arm pivots downwards for the less restrictive (known as "off") indication. Upper quadrant signals, as the name implies, pivot the arm upward for "off".
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. From the 1920s onwards, upper quadrant semaphores almost totally supplanted lower quadrant signals in Great Britain, except on former GWR lines. The advantage of the upper quadrant signal is that should the signal wire break, or the signal arm be weighed down by snow (for instance), gravity will tend to cause the signal to drop to the safe "danger" position. In a lower quadrant signal, the opposite may happen, sending the signal to "off" when in fact it should be illustrating "danger". Counterweights were thus incorporated into lower quadrant signals to prevent this situation arising.
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.
A later development was the upper quadrant three-position semaphore signal. These worked in the upper quadrant to distinguish them from the then standard two-position lower quadrant semaphores. When the arm is inclined upwards at 45 degrees, the meaning is "caution" and the arm in the vertical position means "clear". Thus, three indications can be conveyed with just one arm and without the need for a distant arm on the same post. The early abandonment of the three-position semaphore signal in the UK allowed for the widespread replacement of two-position lower quadrant signals by two-position upper quadrant signals.
Where particularly poor background contrast cannot be avoided, a sighting board (usually painted white) can be placed behind the arm to provide adequate contrast which improves visibility. Alternatively, part of a bridge abutment conveniently located behind a signal may be painted white.
From 1872, distant signal arms (see below) were distinguished by having a "fishtail" notch cut into the end. All signal arms were still painted red at this time. In the 1920s, the British railway companies began to colour their distant signal arms yellow to better distinguish them from the red stop signals. The red lenses in distant signals were changed to yellow at the same time. The practice of using red distant arms has survived in some countries however.
A third type of arm with a pointed end extending outward is often used to indicate that the signal operates automatically (as in automatic block signalling systems) and in many cases indicates that the signal is 'permissive', and can be passed when showing its most restrictive aspect, at a low speed.
For most types of semaphore arm, the colour of light shown when the arm is in the "on" (most restrictive) position generally corresponds to the colour of the arm itself (i.e. red for a stop signal, yellow for a distant signal). When the arm is in the "off" position, a green light is displayed.
Other shapes and colours of semaphore arms were used for specific purposes in different countries around the world.
Semaphore signals may be operated by electric motors or hydraulically, allowing them to be located further from the controlling signal box, or in some cases to work automatically. 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. For lower quadrant semaphores this requires special counterweights to cause the arm to rise rather than fall; this is one of the reasons for the widespread switch to upper quadrant signals.
A stop signal or absolute signal (US English) is any signal whose most restrictive indication is 'danger' (which compels a stop). Stop signals are used to protect junctions, points ('switches' in American terminology), level crossings, movable bridges, platforms, or block sections.
A particular signal box may control one or more stop signals on each running line. In a traditional mechanically signalled area, it is most common for a signal box to have two stop signals governing each line. The first reached by a train is known as the home signal. The last stop signal, known as the starting or section signal, is usually located past the points etc. and controls entry to the block section ahead. The distance between the home and starting signals is usually quite short (typically a few hundred yards), and allows a train to wait for the section ahead of it to clear without blocking the line all the way back to the previous stop signal.
At some locations, more than one home signal might be provided on the same line. These will be identified by names such as outer home and inner home, or home 1, home 2 etc.
An advanced starting signal might be used at a location where it might be desirable to advance a train from a station platform before the section ahead becomes available. In this scenario, the starting signal permits the train to draw forward from the station area toward the advanced starting signal, which controls entry to the section ahead.
If any of the signals beyond the first stop signal are at 'danger', the previous signals will also be held at 'danger' until the train is almost at a stand, to indicate to the driver that the next signal is at 'danger'. This can be enforced by instruction or by electrical interlocking, which requires the provision of a track circuit on the approach to the signal.
In North America, the foregoing terminology was not used, as the development of American signalling practice diverged from that in the United Kingdom during the late nineteenth century. In America, where the term home signal is in common usage, it generally refers to the "generic" British definition of 'stop signal', namely any signal whose most restrictive indication is 'danger'.
A signal that provides advance warning of a stop signal ahead (and which does not compel a stop when in its most restrictive position) is referred to as distant signal. The term originated in British English and is used throughout the English-speaking world. In some regions, notably North America, the terms distant signal and approach signal are both in common usage.
Because of the long distance required to bring a moving train to a stand, distant signals must be located on the approach to the corresponding stop signal by at least the braking distance of the worst braked train to use the route. This is particularly important on high speed routes. At one time it was practice to take sighting distance into account when positioning distant signals; the distant signal could therefore be positioned at less than braking distance to the corresponding stop signal.
The driver of a train encountering a distant signal at 'caution' must expect the stop signal to be at 'danger' and must adjust the train's speed so as to bring the train to a stand before reaching it. The driver of a train encountering a distant signal in the 'clear' position knows that all applicable stop signals controlled by the same signal box are in the 'clear' position.
This is enforced by interlocking; the distant signal is prevented from assuming the 'clear' position unless all relevant stop signals controlled by the signal box display 'clear'.
Where signals are closely spaced, a stop signal and a distant signal can be mounted on the same post. The distant signal is always the lower of the two. The two signals are "slotted" so that the distant signal can only clear if the stop signal is clear. Both signals display a light at night, which means that the 'danger' indication appears as red over yellow.
Mechanical signals worldwide 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 and the new European system ERTMS). In the United States, their usage has been almost completely eliminated on main lines, with significant remaining installations still found on two separate segments of the CSX Transportation line from Louisville, Kentucky to Chicago, Illinois, on the route of the former Monon Railroad.
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