Three Position Signals

Three position signals were introduced as part of the suburban electrification program. They permit trains to follow at much closer intervals than was previously possible, a side benefit was the abolition of many signal boxes.

There are four types of 3 position signals

First of all the term "three position signals" is actually a meaningless term as it simply refers to the number of positions a single arm, or light, can display. Three position signals always need two arms or lights to display their aspects (except in the case of dwarf signals) and in Victoria a total of 6 aspects can be displayed by the combination of the two lights (7 if low speed aspects are included)

Interlocked 2 position signals told a driver exactly which track the points were set for and this convention was carried over to 3 position signals at first. As introduced in 1915, the top arm or light told the driver the points were set for the straight and the bottom arm or light told the driver the points were set for the diverge. In 1918 this convention was abandoned with the introduction of 3 position signals between Newmarket and Kensington.

It is important to understand that 3 position home and automatic signals require 2 lights to display their indications and that aspects and their meanings are identical for both home and automatic signals.

  • A "top light" (green or yellow displayed on the top light) means that a driver can travel at "normal speed" which is the maximum speed laid down for the locality.
  • A "bottom light" (green or yellow on the bottom light) means a driver must not exceed 40 KPH (high speed diverge indicators (illuminated 65's) will be explained later)

Thus, VR 3 position signals are purely speed signals, they do not normally convey information about the route to be taken. Of all the 3 position signals in Victoria only a very small number actually indicate the route to be taken without the use of route indicators, this is why the ad hoc use of route indicators is so widespread.


Identifying 3 position home and automatic signals

Home signals are identified by the 2 lights being vertical.

A semaphore home signal was identified by a red square ended arm

Automatic signals are identified by the 2 lights being staggered.

A semaphore automatic signal was identified by a red pointed arm

I will first go through all of the 3 position aspects and indications and then explain how they are used with diagrams.

In signalling terms an ASPECT is the Appearence of the signal. An INDICATION is the information conveyed by the Aspect.

SEMAPHORE
ASPECT
LIGHT
ASPECT
Indication

"Enginemans name"

next signal (under normal conditions)
G/R
Clear Normal Speed

"Top Green"

Train can proceed at the maximum speed for the locality. Driver can expect the next signal to be displaying a normal speed aspect G/R

Y/G

Y/R

.
Y/R
Normal Speed Warning

"Top Yellow"

Train can proceed at the maximum speed for the locality but driver must be prepared to find the next signal at stop R/R
.
Y/G
Reduce to Medium speed

"Reduce to medium"

Train can proceed at the maximum speed for the locality but must be prepared to pass next signal at 40 KPH. R/Y or R/G
.
R/G
Clear Medium Speed

"Bottom green"

Train can proceed at a maximum of 40 km/h and can expect the next signal to be displaying a proceed aspect. G/R, Y/R, Y/G, R/G, R/Y
R/Y
Medium Speed Warning

"Bottom yellow"

Train can proceed at a maximum of 40 km/h but driver must be prepared to find the next signal at stop R/R
R/R
Stop

"On the red"

At other than the stop indication, the aspects on a home or automatic signal are identical, at the stop position an automatic can be passed after complying with Regulation 74, (now know as rule 1 section 3)
A home signal cannot be passed at stop without the required authority.
R/R/Y
Low Speed Caution

"Low Speed"

Points are set for the move but track may be occupied, maximum of 15 km/h
Let's look at a very common signal aspect sequence in use in the suburban area.
Post 35 at St.Albans protects a set of facing and trailing points. With post 35 at "stop" the automatic before it, post M603 is also held at "stop". The driver is warned about M603 being at stop by a normal speed warning being displayed on post M573. So a driver approaching on clear normal speed aspects only gets one warning (M573) before the red automatic (M603)

ALL DIMENSIONS ARE IN METRES

In areas where higher traffic densities are anticipated (in this case Newmarket to Kensignton when show or race trains are operating in addition to normal Broadmedows suburban and North east country trains) the signals need to be closer together but this would cause a problem because a driver may not get enough advance warning of a signal at stop to be able to stop at it. The solution is to give the driver an additional warning signal. In the case of Kensington an up train, (travelling from right to left) sees E188 which is telling him to slow down to 40km/h at E180, E180 allows him to approach E172 at 40 km/h. Effectively he has 242 metres + 195 metres (437 metres) warning that E172 is at stop, compare this to Ginifer to St albans where the driver gets 873 mteres of advance warning.
It is important to note that the speed between Ginifer and St Albans is 95 km/h compared to Kensington to Newmarket which is 65 km/h

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