Australian railway signalling

Australian railway signalling

Just as the railways of Australia has suffered issues from incompatible rail gauges, the different states have tended to go their own way regarding railway signalling practice.

New South Wales

The signalling system in New South Wales consists currently of four types of signals. The four types form a consecutive line of development. The oldest still in use is the "Lower Quadrant Semaphore" signalling system. The next in line of development is the "Upper Quadrant Semaphore" system, which directly led to the "Double Light Colour Light" system, and this in turn was modified to the "Single Light Colour Light" system.

Double Light signals are capable of showing more indications than Single Light signals, therefore Double Light signals are more appropriate for use in dense traffic areas, such as the Sydney metropolitan area, and Single Light signals for the less intense areas. However combinations of the different types may be found at the same interlocking locations.

Lower quadrant semaphore

Lower Quadrant Semaphore signals use an arm that works in a horizontal position and may be lowered to (about) a 45 degree angle, they can only give two indications. In the horizontal position a red light is displayed, in the lowered position a green light is displayed. There are two types of arms used. A Distant signal uses a fishtail arm, Home and Starting signals use a square-tail arm. The crucial element in the later development of the NSW signalling system was the addition of an upper green marker light, with no attached arm, above the light of the Distant signal of the Lower Quadrant type. This green light was permanently lit thus giving a double light indication in advance of a Home signal.

If the Home or Starting signal was at Stop, then the Distant signal showed a green light over a red light, if the Home and Starting signals were Clear, then the Distant would show two green lights. During daylight the position of the arm of the Distant signal gave the main indication warning of the aspect of the Home or Starting signal.

Therefore as a train approaches an interlocking the first signal will be a Distant. The Distant will show Clear (two green lights) if the road is set through the interlocking, that is if all signals including the Starting signal are Clear. The Distant will show Caution (a green over a red light) if any of the Home or Starting signals is at Stop.

Combined home & distant signals

Where interlockings are closely placed, a Starting signal arm may be fitted above a Distant signal arm. In this case the Starting arm can be placed at Stop and display a red light. As the lower Distant signal arm cannot be cleared while the upper arm is at Stop the signal will show two red lights. The basis for Double Light signalling was thus established by a sequence of Distant and two-arm signals, so that it was then possible to encounter a signal showing two green lights (Clear), followed by a green light over a red light (Caution) then one or two red lights (Stop). This sequence gave a definite pattern as the basis of advance warning signalling.

Upper quadrant signals

Upper Quadrant signals operate with double light indications (though see below concerning Single Light Colour Light signals). They appear as square-tail Distant arms with an upper green "marker" light, or as square-tail Home/Starting arms with a lower "marker" light that can change between red and green as required. The Distant signal only operates in the upper positions giving Caution and Clear indications and does not operate horizontally as it does not give a Stop indication. The Home/Starting signals give three indications. The arm moves from horizontal displaying a red light, upwards to 45 degrees displaying a green light, and fully vertical also displaying a green light. The lower light of these signals stays red until a full Clear indication is shown. Therefore these signals will show the same lights indicated by the Lower Quadrant Semaphore Distant and two-arm signals, that is, two greens for Clear, a green over a red for Caution and two reds for Stop. Again during daylight the arm gives the main indications.

Double light colour light

The step to Double Light Colour Light signals is simply a matter of removing the arms and brightening the lights to be visible in daylight. The sequence remained two green lights for Clear, green over red for Caution, and two red lights for Stop.

As traffic and speed of trains increased additional indications became necessary. The first of these was the introduction of a subsidiary signal: a small green light under the two red lights of a Stop signal. As a running signal this indicates Low Speed. This signal occurs between a Caution indication and a signal at Stop. It allows a further block section available in advance of a signal at Stop protecting a train.

With the development of an unambiguous shade of yellow, further indications became possible. A lower yellow light below an upper green light was introduced as a Medium indication which is used in advance of a Caution signal, giving another block section warning. Another indication of Preliminary Medium is now available with a pulsating yellow light beneath an upper green. It is used before a Medium indication giving yet another protective block warning.

So now the full sequence of indications given by Double Light Colour Light signals is two greens for Clear, a top green and lower pulsating yellow for Preliminary Medium, an upper green over a lower yellow for Medium, a top green and lower red for Caution, two reds with a small subsidiary green for Low Speed and then two red lights for Stop.

A yellow light may also be used as an upper light to indicate turn-out route indications. A top yellow over a red light is Caution Turnout indicating the next signal is at Stop, a top yellow over a bottom yellow is a Clear Turnout signal indicating that the next signal is not at Stop. Double Light Colour Light signals are therefore able to give eight indications.

Preliminary medium indications and turnout repeaters

Junction indicators have been introduced to resolve the ambiguity that may arise when a medium aspect (green over yellow) can precede either a caution aspect (green over red) or one of the turnout aspects (yellow over red/yellow). The junction indicator is a white bar, inclined towards the turnout, placed above the medium aspect. The bar is not lit when the next signal applies to the straight route, but is illuminated when the next signal applies to the turnout. This innovation as yet does not cover all places where it could be used effectively (for example from the up main north to the relief road at Rhodes in Sydney).

Single light colour light

With the replacement of older signals in areas with less traffic, Single Light Colour Light signals were developed. Using a single green light for Clear and a single yellow light for Caution, these signals use a single red light with a smaller lower red "marker" light for Stop. Turn-out indications can be provided with three subsidiary yellow lights under a red Stop light inclined in the direction of the route away from the main route.

Some Upper Quadrant Semaphore signals were adapted as Single Light Colour Light signals giving the same colour indications while retaining the arm. The sequence of indications of Single Light Colour Light signals is one green light for Clear, one yellow light for Caution, one red light with a lower smaller red light for Stop. The last Upper Quadrant Semaphore Single Light signal seems to have been one on the up south main at Moss Vale which was replaced in 2007 with the resignalling and decommissioning of Moss Vale and Moss Vale Junction signal boxes.


The signalling system used in New South Wales has been a progression from Distant and Home signals which is still obvious in the latest Double Light system. The upper light can be read as the Home indication, that is the block section immediately ahead, and the lower light indicates the Distant aspect of the next signal. The Single Light system is a modification of that idea, of course it is also a standard for many signalling systems around the world including other Australian systems.

Contradictory meanings

Signalling systems vary between the states of Australia as each railway was established under the different colonial governments with separate legislation. As with the notorious situation of having different gauges, there are differing signal systems. The Victorian railways use Speed Signalling. This has led to similar signal indications giving very different meanings in these two states. For example, in New South Wales green over red means Caution indicating the next signal is at Stop. In Victoria that same aspect, green over red, means Clear Normal Speed indicating the next signal is anything but at Stop. There is obvious potential for confusion. Drivers who traverse the border must mentally switch off one set of rules and switch on the other.

On the main south line from Sydney, Single Light Colour Light signals are now exclusively used from Spring Creek bridge (south of Galong) to Albury on the Victorian border. This forms a buffer zone between the areas giving conflicting signal indications. Gradually the remaining Upper Quadrant signals (and Double Light Colour Light signals at Binalong) will be replaced by Single Light Colour Light signals.


The railways of Victoria use a mix of railway signalling practices: British route signalling with home and distant signals ("2 position signalling") and American speed signalling ("3 position signalling").cite book
last = Fisher
first = Peter
title = Victorian Signalling: by Accident or Design?
publisher = Australian Railway Historical Society (Victorian Division)
year = 2007
page = page "x"
isbn = 978 1 92089 250 0
Semaphore signals were used on the very first railway lines, but only a bare minimum were provided as the time interval system being relied upon instead.cite book
last = Fisher
first = Peter
title = Victorian Signalling: by Accident or Design?
publisher = Australian Railway Historical Society (Victorian Division)
year = 2007
page = page 39
isbn = 978 1 92089 250 0
] The first interlocking of signals to protect trains was provided in 1874, cite web | title = VR timeline | work = | publisher = Mark Bau | url = | format = HTML | accessdate = 2008-02-05 ] as before this time conflicting moves could be made. The design of the signals also progressed, with the "disc" type siding signals first introduced in 1885,cite book
last = Fisher
first = Peter
title = Victorian Signalling: by Accident or Design?
publisher = Australian Railway Historical Society (Victorian Division)
year = 2007
page = page 62
isbn = 978 1 92089 250 0
] and the lower quadrant "somersault" type main line signals adopted in 1887, both of which are still in use today. Green was not adopted as the "All Right" colour until 1898, with white being used before this time. Red was the usually colour of all signal arms, until yellow was chosen as the colour for distant signals in 1926, with full adoption made in 1930. Colour light signals first appeared in 1918, and by 1924 they were the standard for new installations. The safeworking of trains between stations on the early lines was time interval working, where a train would be allowed to leave a given time after the train before it. With heavier traffic this method became unsafe, with Staff and Ticket working on single lines adopted from 1873, and telegraph block working from 1878 on double lines.cite book
last = Fisher
first = Peter
title = Victorian Signalling: by Accident or Design?
publisher = Australian Railway Historical Society (Victorian Division)
year = 2007
page = page 64
isbn = 978 1 92089 250 0
] Both of these systems ensured that only one train would be in a section of track at one time. Telegraphic block working was then replaced with "Winters Block" working between 1883 and 1888,cite book
last = Fisher
first = Peter
title = Victorian Signalling: by Accident or Design?
publisher = Australian Railway Historical Society (Victorian Division)
year = 2007
page = page 66
isbn = 978 1 92089 250 0
] a system that is a predecessor of the "Double Line Block" system which is still used today. Later years saw variations made to the "Staff and Ticket" system, with busier lines provided with Electric Staff working which provided greater safely when more trains ran.

Heavier suburban traffic on the Melbourne network saw a greater stain on the block working then used, which required a large number of manned signal boxes to enable trains to run close together.cite book
last = S.E. Doorman and R.G. Henderson
title = Electric Railways of Victoria
publisher = Australian Electric Traction Society
page = page 20
year = 1979
isbn = 0 909459 06 1
] As a result it was decided to adopt power signalling under the Automatic Block System (ABS) of safeworking, where the presence of trains automatically control the signals after them, providing a safe distance between trains. Introduced from 1915, the system was based on American speed signalling practice with GRS2A upper quadrant mechanical signals with two arms able to indicate up to 5 different speed aspects to train drivers. These signals were later replaced by colour light signals which are the standard today, but the old mechanical style remained until 2001.cite book
last = Fisher
first = Peter
title = Victorian Signalling: by Accident or Design?
publisher = Australian Railway Historical Society (Victorian Division)
year = 2007
page = page 89
isbn = 978 1 92089 250 0
] A variant of the "Automatic Block System", "Automatic and Track Control" (ATC) has since been introduced, which provides the same benefits as ABS on single lines of track, while still ensuring only one train in a section at a time. Centralised Traffic Control was also introduced in the 1960s on the new standard gauge line to Albury,cite web
title = Somerton
work = Victorian Station Histories
author = Andrew Waugh
url =
format = PDF
accessdate = 2008-02-08
] and then on the main interstate line to Adelaide, allowing trains to be directed from a distance.

Today little mechanical signalling remains, with local signal boxes controlling signals abolished from many areas as part of the Regional Fast Rail project. Today the suburban network and busier regional lines use variants of Automatic Block Signalling, while quieter lines use the Train Staff and Ticket or Train Order systems of safeworking. [cite web | title = Safeworking in Victoria
work = Vicsig | url = | format = HTML | accessdate = 2008-02-08
] Train protection has also progressed, with the Train Protection & Warning System also introduced on major passenger lines.

South Australia

South Australia uses two primary forms of signalling. Nearly all signal boxes in South Australia have now been closed, with most rail traffic is coordinated through centralised traffic control systems, either under Australian Rail Track Corporation control from Mile End or TransAdelaide control from Adelaide. Where these two networks interface, such as at the Goodwood grade crossing or at Torrens Junction, control is usually from ARTC after release from TransAdelaide. Despite the almost uniform CTC control some signal boxes still exist, such as Dry Creek South although they are not normally switched in.

Speed signalling

Before 1988, signalling in the metropolitan area was three position speed signalling, similar to the Victorian system. All mainline signals have two signal 'heads' (originally upper quadrant semaphore arms but now colour light, LED or searchlight), both are always lit. The aspects shown depend on the speed of the route set. If the route was for normal speed, the indication is given by the top head, if the move is for medium speed (40km/h, such as when entering a loop or siding), the indication is given by the lower head. The colours displayed by either depended on how many blocks ahead were clear, green if two or more blocks were clear and yellow if only one was clear (i.e. the next signal showed stop).

Hence the aspects are:
* Red over red: Stop.
* Green over red: Normal Clear.
* Yellow over red: Normal Caution.
* Red over green: Medium Clear.
* Red over yellow: Medium Caution.

In addition to these aspects there was a Yellow over Green aspect for 'reduce to medium speed', used when the next signal showed a medium speed aspect. If either of the two main lights never shows any colour other than red, it is replaced with a permanently lit red marker lamp.

Dwarf signals There is also a low speed aspect for calling-on, shunting or entering a low speed area such as a goods siding. This is indicated by a small yellow light beneath the two main lights, i.e. red over red over yellow.Dwarf signals are also used to show low speed aspects. These were originally upper quadrant disc signals but are now all of the colour light or searchlight variety. The aspects are:

* Single red or purple light: Stop.
* Single yellow light: Low Speed Caution (block may be occupied).
* Single green light: Low Clear (next signal is clear or caution).

Permissive vs absolute Signals are divided into Permissive and Absolute signals. Absolute signals cannot be passed at stop without permission from the signaller, whereas Permissive signals at stop can be passed after waiting one minute. Absolute signals can be identified by the fact that the two lights are aligned vertically, whereas in Permissive signals they are staggered (on different sides of the post). Dwarf signals are always Absolute.

Suburban network

In 1988 the TransAdelaide lines were resignalled with the opening of the Adelaide control centre. The system is quite similar to modern UK colour light signalling, being a route signalling system, though it lacks the double yellow aspect and makes use of 'permissive' signals. There are three basic aspects: red for stop, yellow for caution and green for clear. A reduce to medium speed aspect is also used to give early warning of a divergence, and is given by a flashing yellow light. All aspects are indicated with colour light signals. Despite this re-signalling, some parts of the TransAdelaide network still use the original 3-position speed signalling (such as the Dry Creek to Port Adelaide line).

There is also a low speed aspect indicated by a lunar-white position-light signal mounted below the main head. It shows two lunar-white lights at 45 degrees (to mimic an upper-quadrant semaphore) to indicate proceed at low speed. These position light signals are also used for dwarf signals and in this case they can also display 'stop' as a red light and lunar-white light in a horizontal row.

Junctions on running signals are indicated by a row of five lunar-white lights angled in the direction of divergence. If multiple routes are to be signalled, several rows may be used.

Permissive signals are indicated by a circular 'P' plate offset below the signal head and Absolute by a square 'A' plate directly below the signal head. As with three-position signalling, dwarf (low speed) signals are always absolute. An 'A' plate in New South Wales or Victoria means the signal is automatic and therefore a Permissive signal, which is again a source of conflicting meaning between the differing signalling systems.

In the Adelaide station yard Theatre-style route indicators are used on both running signals and shunt signals; platform numbers for 'up' trains and route through the yard for 'down' trains. The down indicators can show these indications:

* SS-south suburban line,
* SM-south main line,
* PM-port main line,
* NM-north main line,
* RD-rail car depot,
* SY-stabling siding,
* NY-north yard.


Follows UK practice.

Except that flashing yellow allows simultaneous entry into a crossing loop with no overlaps.

Western Australia

Follows a practice similar to UK modern colour light signalling with junctions being signalled with rows of five lunar-white lights (also known as 'feathers'), though there are many colour light junction signals where a signal head of variable height was provided for each route.


Follows UK practice.


* Training manuals of the New South Wales State Rail Authority

External links

* [] Australian Rail Track Corporation website where NSW signalling standards, principles, safeworking rules are available.

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