Railway semaphore signal
Encyclopedia
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.
on the London and Croydon Railway
(later the London Brighton and South Coast Railway) at New Cross
, southeast London, about 1842 on the newly enlarged layout also accommodating the South Eastern Railway. According to J.T. Howard Turner, John Urpeth Rastrick
proposed the idea to Gregory.
The semaphore was afterwards rapidly adopted as a fixed signal throughout Britain, superseding all other types in most uses by 1870. Such signals were widely adopted in the U.S. after 1908.
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 tested 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". 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.
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.
system superseded time-interval working.
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 (though not in the UK) 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.
. Later, remote operation was effected by levers
connected to the signal by wire (UK) or by pipes supported on rollers (US & Australia). The levers were grouped together in a building known as a "signal box" (UK & Australia) or "interlocking tower" (US). In the UK multiple signal wires were grouped together, and in station areas were commonly routed beneath platform faces just above rail level. The operation of a single signal wire would cause some of the adjacent wires to 'jangle', and this noise was one of the many characteristic sounds of railway travel until the gradual introduction of colour-light signals from the mid 20th century.
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 the spectacle case to be sufficiently heavy to ensure the arm rises rather than falls; this is one of the reasons for the widespread switch to upper quadrant signals. (The purpose of the balance weight often found on a lever lower down the post of a semaphore signal is actually to ensure the signal wire returns to its normal position when the controlling lever is put back. Otherwise, again, the signal would fail to return to danger. These counterweights are used just as commonly on upper- as on lower-quadrant signals.)
A stop signal (US - home signal or absolute signal) is any signal whose most restrictive indication is 'danger' (which compels a stop). Stop signals are used to protect junctions
, points
(US - 'switches'), level crossing
s, 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 first home, second home 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 the UK 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.
order. It was common for train order signals to point the arm straight down to indicate 'proceed'.
s) and also to control road traffic (e.g. at level crossing
s).
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
.
In the UK, semaphore signals are still quite common on the rail network on secondary routes, however the increase in spending on renewals is leading to a faster rate of decline in recent years.
Railway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...
is the semaphore. These signals display their different indications to train drivers
Railroad engineer
A railroad engineer, locomotive engineer, train operator, train driver or engine driver is a person who drives a train on a railroad...
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.
Origins
The first railway semaphore signal was erected by Charles Hutton GregoryCharles Hutton Gregory
Sir Charles Hutton Gregory KCMG was a British civil engineer. He was president of the Institution of Civil Engineers between December 1867 and December 1869....
on the London and Croydon Railway
London and Croydon Railway
The London and Croydon Railway was an early railway which operated between London and Croydon in England. It was opened in 1839 and in July 1846 it merged with other railways to form a part of the London Brighton and South Coast Railway ....
(later the London Brighton and South Coast Railway) at New Cross
New Cross
New Cross is a district and ward of the London Borough of Lewisham, England. It is situated 4 miles south-east of Charing Cross. The ward covered by London post town and the SE 14 postcode district. New Cross is near St Johns, Telegraph Hill, Nunhead, Peckham, Brockley, Deptford and Greenwich...
, southeast London, about 1842 on the newly enlarged layout also accommodating the South Eastern Railway. According to J.T. Howard Turner, John Urpeth Rastrick
John Urpeth Rastrick
John Urpeth Rastrick was one of the first English steam locomotive builders. In partnership with James Foster, he formed Foster, Rastrick and Company, the locomotive construction company that built the Stourbridge Lion in 1829 for export to the Delaware and Hudson Railroad in America.-Early...
proposed the idea to Gregory.
The semaphore was afterwards rapidly adopted as a fixed signal throughout Britain, superseding all other types in most uses by 1870. Such signals were widely adopted in the U.S. after 1908.
Components
The semaphore 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. Usually these were combined into a single frame, though in some types (e.g. "somersault" signals in which the arm pivoted in the centre), the arm was separate from the spectacle. The arm projects horizontally in its most restrictive aspect; other angles indicate less restrictive aspects.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
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...
clusters have also been tested for this purpose). Where a green light was required, a blue lens would usually be used. When combined with the mainly yellow-emitting flame
Flame
A flame is the visible , gaseous part of a fire. It is caused by a highly exothermic reaction taking place in a thin zone...
of an oil lamp
Oil lamp
An oil lamp is an object used to produce light continuously for a period of time using an oil-based fuel source. The use of oil lamps began thousands of years ago and is continued to this day....
, this produced a green colour. Later signals using electric lamps
Electric light
Electric lights are a convenient and economic form of artificial lighting which provide increased comfort, safety and efficiency. Most electric lighting is powered by centrally-generated electric power, but lighting may also be powered by mobile or standby electric generators or battery systems...
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
Southern Railway (Great Britain)
The Southern Railway was a British railway company established in the 1923 Grouping. It linked London with the Channel ports, South West England, South coast resorts and Kent...
in Great Britain frequently made use of old rail
Rail tracks
The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...
for signal posts.
Lower quadrant and upper quadrant
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
Great Western Railway
The Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...
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". 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.
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.
Two-position and three-position signals
The first railway semaphore signals had arms that could be worked to three positions, in the lower quadrant. Used in conjunction with the time-interval system, the arm horizontal meant "danger", inclined downwards at 45 degrees meant "caution" and the arm vertical (arm hidden within the post) meant "clear". The vertical indication gradually came to be discontinued as the absolute blockBritish absolute block signalling
The principle of the British absolute block system of railway signalling is to ensure the safe operation of a railway by allowing only one train to occupy a defined section of track at a time...
system superseded time-interval working.
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.
Colouring and shape of arm
Red was quickly chosen as the best colour for semaphore arms as it was clearly visible against most backgrounds. To enhance the visibility of the arm, a marking of contrasting colour, such as a stripe or spot, is usually applied. The rear of the arm is usually coloured white with a black marking.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 (though not in the UK) 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.
Operation
The first semaphore signals were controlled by levers situated at the signals. Some early signals protruded from the roof of the controlling signal boxSignal box
On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetable...
. Later, remote operation was effected by levers
Lever frame
Mechanical railway signalling installations rely on lever frames for their operation to interlock the signals and points to allow the safe operation of trains in the area the signals control...
connected to the signal by wire (UK) or by pipes supported on rollers (US & Australia). The levers were grouped together in a building known as a "signal box" (UK & Australia) or "interlocking tower" (US). In the UK multiple signal wires were grouped together, and in station areas were commonly routed beneath platform faces just above rail level. The operation of a single signal wire would cause some of the adjacent wires to 'jangle', and this noise was one of the many characteristic sounds of railway travel until the gradual introduction of colour-light signals from the mid 20th century.
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
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
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 the spectacle case to be sufficiently heavy to ensure the arm rises rather than falls; this is one of the reasons for the widespread switch to upper quadrant signals. (The purpose of the balance weight often found on a lever lower down the post of a semaphore signal is actually to ensure the signal wire returns to its normal position when the controlling lever is put back. Otherwise, again, the signal would fail to return to danger. These counterweights are used just as commonly on upper- as on lower-quadrant signals.)
Stop signals
Junction (rail)
A junction, in the context of rail transport, is a place at which two or more rail routes converge or diverge.This implies a physical connection between the tracks of the two routes , 'points' and signalling.one or two tracks each meet at a junction, a fairly simple layout of tracks suffices to...
, points
Railroad switch
A railroad switch, turnout or [set of] points is a mechanical installation enabling railway trains to be guided from one track to another at a railway junction....
(US - 'switches'), level crossing
Level crossing
A level crossing occurs where a railway line is intersected by a road or path onone level, without recourse to a bridge or tunnel. It is a type of at-grade intersection. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion...
s, movable bridges, platforms
Railway platform
A railway platform is a section of pathway, alongside rail tracks at a train station, metro station or tram stop, at which passengers may board or alight from trains or trams. Almost all stations for rail transport have some form of platforms, with larger stations having multiple 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 first home, second home 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
Interlocking
In railway signalling, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as junctions or crossings. The signalling appliances and tracks are sometimes collectively referred to as an interlocking plant...
, which requires the provision of a track circuit
Track circuit
A track circuit is a simple electrical device used to detect the absence of a train on rail tracks, used to inform signallers and control relevant signals.- Principles and operation :...
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'.
Distant signals
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
Braking distance
Braking distance refers to the distance a vehicle will travel from the point where its brakes are fully applied to when it comes to a complete stop...
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'.
Combined stop and distant signals
Shunting and subsidiary signals
Shunting signals and subsidiary signals also exist in semaphore form, with smaller arms and lights than are provided for main signals. These signals may also take the form of a disc with a horizontal stripe which is rotated 45 degrees to the clear position.Train order signals
In North America, semaphores were employed as train order signals, with the purpose of indicating to engineers whether they should stop to receive a telegraphedElectrical telegraph
An electrical telegraph is a telegraph that uses electrical signals, usually conveyed via telecommunication lines or radio. The electromagnetic telegraph is a device for human-to-human transmission of coded text messages....
order. It was common for train order signals to point the arm straight down to indicate 'proceed'.
Non-railway uses
Railway-style semaphore signals have been used to control movements of boats or ships (e.g. at swing bridgeSwing bridge
A swing bridge is a movable bridge that has as its primary structural support a vertical locating pin and support ring, usually at or near to its centre of gravity, about which the turning span can then pivot horizontally as shown in the animated illustration to the right...
s) and also to control road traffic (e.g. at level crossing
Level crossing
A level crossing occurs where a railway line is intersected by a road or path onone level, without recourse to a bridge or tunnel. It is a type of at-grade intersection. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion...
s).
Decline in usage
Radio Electronic Token Block
Radio Electronic Token Block is a system of railway signalling used in the United Kingdom. It is a development of the physical token system for controlling traffic on single lines.- How it works :...
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
CSX Transportation
CSX Transportation operates a Class I railroad in the United States known as the CSX Railroad. It is the main subsidiary of the CSX Corporation. The company is headquartered in Jacksonville, Florida, and owns approximately 21,000 route miles...
line from Louisville, Kentucky
Louisville, Kentucky
Louisville is the largest city in the U.S. state of Kentucky, and the county seat of Jefferson County. Since 2003, the city's borders have been coterminous with those of the county because of a city-county merger. The city's population at the 2010 census was 741,096...
to Chicago, Illinois, on the route of the former Monon Railroad
Monon Railroad
The Monon Railroad , also known as the Chicago, Indianapolis and Louisville Railway from 1897–1956, operated almost entirely within the state of Indiana...
.
In the UK, semaphore signals are still quite common on the rail network on secondary routes, however the increase in spending on renewals is leading to a faster rate of decline in recent years.
See also
- Signal passed at dangerSignal passed at dangerA Signal passed at danger , in British railway terminology, occurs when a train passes a stop signal without authority to do so. It is a term primarily used within the British Railway Industry, although it can be applied worldwide.-Categories of SPAD:...
- Rule 55Rule 55Rule 55 was an operating rule which applied on the former British Railways in the 19th century, and was made defunct when the was introduced following privatisation of the railway.- Overview :...
- Pass of Brander stone signalsPass of Brander stone signalsThe Pass of Brander stone signals are a series of railway signals situated in the Pass of Brander, between and stations on the Oban branch of the West Highland Line in Scotland. They are part of a warning system that advises train drivers to exercise caution in the event of a rock-fall.The...
- Semaphore line