This has been argued for eons by signal professionals/enthusiasts. On the
one hand it does not make sense to have a red light as part of your best
proced aspect, (Vic G/R) OTOH the NSW system does allow some strange
indications if certain lights fail and go dark.
It should be pointed out that NSW adopted a system of indications that was
unique whereas Vic and SA's system is a very common system used throughout
Nth Ameica.
NSW system basically just took the aspect of distant and home signals and
converted them into lights. For instance, in both Vic and NSW when a 2
position home and distant are on the same post the light indications are
either R/R (stop) - G/R (home at proceed, distant telling you next signal
may be at stop) or G/G, (both home and distant at proceed, next signal at
proceed.) These are pretty much what current NSW signals tell you.
The system used by the VR was so bastardised over the years that it bears
little resemblance to the system it was modeled after. When introduced, a
top light meant you were going straight and a bottom light meant the
points were in reverse. No ifs or buts! This is very clearly spelled out
in the circular that accompanied the systems introduction, (of which I
have a photocopy) When it was desired to reduce headways in areas that had
both sparks (short stopping distance) and loco hauled trains (longer
stopping distance) A single Y/R (normal speed warning) was deemed to be
insufficient for loco hauled trains due to the short distance between
signals. So they came up with speed signalling.
No longer did R/Y or R/G mean you were diverging, it simply told you to
slow to a diverging speed. With this system, instead of getting a Y/R
coming up to a red signal you know got Y/G (reduce to medium) - R/Y
(medium speed warning) this gave loco hauled trains more time to prepare
to stop. With the introduction of high speed points an illuminated 40
could be attached to the signal displaying Y/G as well as the signal that
displayed R/G. Later you could get a 40 on a R/Y.
<<<Anyone know the history or logic if any of these indications?>>>
The history/intent is pretty simple although it has been lost over the
years by constant fiddling by signal engineers. A 3 pos signal tells you
if the next signal is also at proceed (green light) or if the next signal
may be at stop (yellow light) the POSITION of the green light or yellow
light tells you what speed you may proceed at. To tell you the next signal
is at a lower speed than track speed Y/G (reduce to medium) is used. On a
G/R the medium speed light serves no purpose, except to show you that the
green is in fact the top light.
<<<<Also is there any differances in interpretation between a red over red
indication and a single red,ie single color light signal with no lower
light,as the down departure signal at Sunbury displays.?>>>
1 2 or 3 (only used in NA) red lights have no inherant difference, (at
least in Vic or SA) until you stop. One red light by itself is a home
signal (or starting) as is 2 vertical red lights, staggered red lights
indicate an automatic as does a single red light with a A plate or
illuminated red under a home. In all cases you must stop at the signal
first. Then you act depending if you are stopped at a home or auto.
Mark.
Some people make art to impress others.
I make art to impress myself.