Australian railway signalling
Overview
Just as Australia has suffered from a gauge muddle, the different states have tended to go their own way regarding 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 metroploitan 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 may appear as fishtail 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/Staring signals give three indications. The arm moves from horizontal diplaying 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.
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 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
In New South Wales, Green over Red means Caution.
In Victoria, in 3 position signal territory, Green over Red means Full Speed.
Some trains journeys across state borders encouter both of these contradictory meanings, and there is a small potential for confusion. When the last remaining double light semaphore signals at Junee and Cootamunda are converted to single light indications, this potential source of confusion will be substantially reduced.
Victoria
The Lower Quadrant signals in this state resemble UK practice, except that the distant arm of a combined Home and Distant was red.
The Upper Quadrant signals in this state follow the speed signalling practice from America.
South Australia
Tended to follow Victoria.
Queensland
Follows UK practice.
Western Australia
Follows UK practice, though there are many colour light junction signals were a signal head of variable height was provided for each route.
Tasmania
Follows UK practice.
See also
Sources
- Training manuals of The New South Wales State Rail Authority