THE SIGNAL BOX

OVERSEAS

NSW DOUBLE LIGHT COLOUR LIGHT SIGNALS

by John Hinson and Adam Kinghi

Multi-aspect, multi-light colour-light or Double Light Colour Light signals are used in the Sydney and Newcastle metropolitan areas, and most indications they give correspond to the standard colour-light route signalling system.

This system was a logical development from the semaphore system, where (like in the UK before 1923) semaphore signals with a home and distant arm would show (by night):

STOP Red over Red
CAUTION Green over Red
CLEAR Green over Green

In New South Wales, the introduction of colour light signals (and the extra yellow colour) was made to be compatible with existing semaphores, whereas in the arrangement adopted in the UK has, to this day, a number of compatibility issues. Junction signals in NSW originally had separate heads, like the semaphore equivalents

It was some years later that the potential fourth aspect was made possible by the bottom yellow light, and for a junction signal to be made possible with a top yellow light.

Double Light Colour Light signals have two colour-light heads, one above the other on a post. The top usually has two lights - green above a red, and the lower head has three or four - green, yellow, red, and sometimes another small green light at the bottom.

The upper head is like a two aspect block signal and gives a clear or danger indication by a green or red light.

The lower head directly indicates to the driver the indication of the next signal ahead (it gives the degree of caution - but a red light indicated by it does not mean stop - it means that the next signal is showing red. A yellow on the lower head does not mean caution - it means that the next signal is indicating caution...i.e. it means Advance Caution.

The illustration shows the possibilities:

  Indication Meaning
Automatic signal with Low Speed indication RED over RED STOP
GREEN over RED CAUTION - next signal at STOP
GREEN over YELLOW MEDIUM - next signal at CAUTION
GREEN over FLASHING YELLOW ADVANCE - next signal at MEDIUM
(provided in closely signalled areas, introduced 2001)
GREEN over GREEN CLEAR
RED over RED over SMALL GREEN LOW SPEED - proceed at not more than 17 mph/27 km/h
(only provided where locations require it)

Junction signals

Where a signal controls a route involving a facing turnout or junction, a three aspect upper head will be provided, showing a green, yellow and red light. The lower head would be as described above. The upper yellow light is only illuminated when the route is set for the turnout. A further warning of the indication of the next signal beyond the turnout is given by the lower head, which will only indicate yellow, meaning the next signal is not indicating stop, or red, meaning the next signal is indicating stop. When the route is for straight ahead, the upper yellow light is not used and the signal works normally as shown above. This is a form of speed signalling for the signal only defines between speed of routes.

  Indication Meaning Route
Double Light Colour Light junction signal RED over RED STOP  
GREEN over RED CAUTION - next signal at STOP Main
GREEN over YELLOW MEDIUM - next signal at CAUTION Main
GREEN over FLASHING YELLOW ADVANCE - next signal at MEDIUM
(provided in closely signalled areas, introduced 2001)
Main
GREEN over GREEN CLEAR
can also be used at high-speed junctions for divergences
Main
YELLOW over RED MEDIUM CAUTION - medium speed through junction, next signal at STOP Divergence
YELLOW over YELLOW MEDIUM TURNOUT - medium speed through turnout, next signal is not at STOP Divergence
Note that YELLOW OVER FLASHING YELLOW, and YELLOW over GREEN are not used
RED over RED over SMALL GREEN LOW SPEED - proceed at not more than 17 mph/27 km/h
(only provided where locations require it)
All

Alternatively a theatre (matrix) type route indicator would be provided above a single three aspect head.

Comparisons

  Four aspect NSW Double Light Colour Light
CLEAR GREEN GREEN over GREEN
ADVANCE CAUTION DOUBLE or FLASHING YELLOW GREEN over YELLOW
CAUTION YELLOW GREEN over RED
DANGER (STOP) RED RED over RED
DRAW FORWARD TO NEXT SIGNAL prepared to stop (max17 mph/27 km/h) No equivalent RED, over RED over SMALL GREEN.

Route Indicators

Around 1995, Route Indicators started to appear to resolve the fact that the medium indication "GREEN over YELLOW" which could mean either next signal at caution, or next signal showing a turnout aspect. It looks like a British "feathers" junction indicator, but is located at the distant signal - one, two (most commonly) or three signals before the junction. Strictly speaking, the Route Indicator should be mounted above the lower lampcase, since it is a distant function, but as there is no room it is mounted above the home lampcase at the top of the signal.

Automatic signals

Automatic signals have the marker lamp or lower signal head offset, to make their function clear, whereas controlled and semi-automatic signals have them in line. Trains are pernitted to pass an automatic signal showing Stop after waiting one minute, but must proceed at a speed such as to be able to stop short of any intermediate obstruction. Normal speed cannot be resumed until a following signal is passed in a clear position.

Conventional colour-light signals

From the late 1950s, simpler signalling using a single head with aspects similar to UK practice was introduced in NSW, for reasons of economy and compatibility with the signalling of other states. Nevertheless, large areas of the Double Light signals remain.

Marker lights

A small red lamp situated below a normal three aspect colour-light head is a marker lamp and acts like a repeater for the main red lamp, and is illuminated at the same time, and as a backup in case of bulb failure. So you'll always have two red lights for stop.

There is no need for a secondary marker light on Double Light Colour Light signals as there is already a second red aspect.

Additional notes by Anthony Koch, Peter Neve and Zak Russell



Comments about this article should be addressed to John Hinson