"Five" aspect signalling

As stated, it's due to braking distances. The Great Eastern Main Line has loads of them too, up to three double yellows in sequence I believe.

In a discussion in a different forum about consecutive double yellows....


" said:

As stated, it's due to braking distances. The Great Eastern Main Line has loads of them too, up to three double yellows in sequence I believe.

This begs the question.....

In the Sydney metro area, the equivalent to a UK Double Yellow is Green over Yellow (called "Medium"). For the same reasons as Geoff mentioned, it is possible to get 2 or 3 consecutive G/Y signals in a row. In recent years the local rail authority introduced a fifth aspect to try and cut down on consecutive G/Y aspects. The new aspect is Green over *flashing Yellow (called "Preliminary Medium - see disclaimer below). To compare with UK signalling, this would be something between a Green and a Double Yellow aspect. This effectively means areas that have the "Preliminary Medium" aspect could be regarded, in UK terms, as having "five aspect signalling".

I know a lot of development has occurred over the years in the UK with regard to lineside signalling, however, has introduction of a fifth aspect ever been looked at?.

DISCLAIMER: Just to keep the NSW signalling people from spontaneously combusting :yikes , the *correct* term is "Pulsating Yellow", so called because the lamp is bright for 640 milliseconds then dim for 200 milliseconds (as opposed to "flashing" which is on/off).

I know what you are trying to achieve, and commend that, but at what point does it cease to be a Signal and become an advert for a Lighting Shop? :side:

The UK had "five" aspect signalling for a time on the ECML (unsure if it's still there) with a flashing green giving Class91+MK4 sets authority to go to 140mph. FG-G-YY-Y-R was the sequence.

The UK had "five" aspect signalling for a time on the ECML (unsure if it's still there) with a flashing green giving Class91+MK4 sets authority to go to 140mph. FG-G-YY-Y-R was the sequence.

" said:

The UK had "five" aspect signalling for a time on the ECML (unsure if it's still there) with a flashing green giving Class91+MK4 sets authority to go to 140mph. FG-G-YY-Y-R was the sequence.

They're still there, but are treated as a normal green. Flashing greens are provided on the Down Fast between P487 and P615, and on the Up Fast between P610 and P494.

In the Sydney metro area, the equivalent to a UK Double Yellow is Green over Yellow (called "Medium"). For the same reasons as Geoff mentioned, it is possible to get 2 or 3 consecutive G/Y signals in a row. In recent years the local rail authority introduced a fifth aspect to try and cut down on consecutive G/Y aspects. The new aspect is Green over *flashing Yellow (called "Preliminary Medium - see disclaimer below). To compare with UK signalling, this would be something between a Green and a Double Yellow aspect. This effectively means areas that have the "Preliminary Medium" aspect could be regarded, in UK terms, as having "five aspect signalling".

I know a lot of development has occurred over the years in the UK with regard to lineside signalling, however, has introduction of a fifth aspect ever been looked at?.

In the Sydney metro area, the equivalent to a UK Double Yellow is Green over Yellow (called "Medium"). For the same reasons as Geoff mentioned, it is possible to get 2 or 3 consecutive G/Y signals in a row. In recent years the local rail authority introduced a fifth aspect to try and cut down on consecutive G/Y aspects. The new aspect is Green over *flashing Yellow (called "Preliminary Medium - see disclaimer below). To compare with UK signalling, this would be something between a Green and a Double Yellow aspect. This effectively means areas that have the "Preliminary Medium" aspect could be regarded, in UK terms, as having "five aspect signalling".

When I was doing research into Preliminary Medium it was mentioned that it is also used to give advance notice of diverging movements as well, specifically diverging movements to stop. It seems apparent that G/*Y* was implemented to clear up the confusion between G/Y as a preliminary caution and as an approach diverging indication. Use in 5 block signaling is a secondary benefit.

Also you forgot the Low Speed indication which can act as a 6th main aspect in the NSW scheme.

" said:

I know a lot of development has occurred over the years in the UK with regard to lineside signalling, however, has introduction of a fifth aspect ever been looked at?.

In the UK a 5th main aspect would only be useful at the margins, allowing drivers to go slightly faster at a few places where consecutive double yellows to a stop are being employed.

I'm sorry, Mike, but once again, you are very wrong. It is the Turnout Repeater that was implemented to rid the confusion of a Green over Yellow aspect.

Therefore, the introduction of the "Preliminary Medium" is exactly what I said it was - the creation of an additional aspect.

Lastly, I'll clear up the confusion I've just created with all you British folk! :doh I can imagine now quite a few of you thinking "Hang on, if they use a feather for a Turnout Repeater, what do they use for the turnout"? The answer? Not a feather!! You can learn all about railway signalling in the Australian state of NSW here

I know a lot of development has occurred over the years in the UK with regard to lineside signalling, however, has introduction of a fifth aspect ever been looked at?.

What came first, Preliminary Medium or the turnout repeaters?

Preliminary Medium was introduced in 1993 with junction repeaters (JP) not appearing until 1997-98. The first turnout repeater was installed on ST407 @ Lidcombe. Ironically it has since been removed from this signal due to junction upgrades.

Preliminary mediums were first installed on the Blue Mountains line which at the time had SLCL signalling.

" said:

I'm sorry, Mike, but once again, you are very wrong. It is the Turnout Repeater that was implemented to rid the confusion of a Green over Yellow aspect.

You might want to fix Wikipedia in that regard.

BTW how short are the signal blocks if 5-block signaling is necessary?

Are there any extended lengths of 5-block or is this used more for special cases?

" said:

Lastly, I'll clear up the confusion I've just created with all you British folk! :doh I can imagine now quite a few of you thinking "Hang on, if they use a feather for a Turnout Repeater, what do they use for the turnout"? The answer? Not a feather!! You can learn all about railway signalling in the Australian state of NSW here

That won't clear up the confusion as Turnout Repeaters are covered in CNSG 604 and the normal signal indications being covered in CNSG 606!

" said:

Preliminary Medium was introduced in 1993 with junction repeaters (JP) not appearing until 1997-98. The first turnout repeater was installed on ST407 @ Lidcombe. Ironically it has since been removed from this signal due to junction upgrades.

Preliminary mediums were first installed on the Blue Mountains line which at the time had SLCL signalling.

So the motivation could have been to remove some ambiguity over diverging movements as was suggested by some Wikipedia editor.

Is 5-block signaling actually helpful otherwise?

Wikipedia? I've seen that entry and, dare I say it, it's a train wreck!! Full of unnecessary (and uncalled for) author opinion and full of errors, including the bit about how Preliminary Medium was introduced to mitigate confusion over the Medium (G/Y) aspect.

Obviously short enough that minimum braking distance is required to be spread over 3 signal block sections. This, of course, would be entirely dependent on the local conditions - line speed, type of traffic, gradients. This is like asking "How long is a piece of string"?

Yes. Moreover, there are extended lengths of straight line railway (IE: no junctions/turnouts) that uses Preliminary Medium as a 5th aspect. I cannot definitively state whether there are any locations that use both the Preliminary Medium AND the Low Speed, creating "6 aspect" signalling. Sparky will be able to comment on this.

Unlike any other state in Australia, NSW, in their rules and procedures, draw a distinct and separate definition between Signals and Indicators, even to the point where they are contained in their own separate manuals. A Signal provides a movement authority. There is nowhere in NSW where a Signal exists within an authority-based safeworking system (IE: Train Order). There are appliances within Train Order territory that LOOK like signals (IE: Main Line Indicators) but, as the name suggests, these are also only Indicators and do not provide a Movement Authority.

An Indicator, on the other hand, provides information. A Turnout Repeater is an Indicator which provides supporting information about the signal it is fixed to. If the Indicator fails, the signal can still display the relevant aspect, however, the Driver will have to return to guessing what the next aspect will be (either "Caution" or "Turnout"). Either way, the worst that will happen is a wrong-routed train.

NO!!!! It has nothing to do with diverging routes and everything to do with eliminating consecutive "Medium" aspects. The Wikipedia entry is wrong.

NO!!!! It has nothing to do with diverging routes and everything to do with eliminating consecutive "Medium" aspects. The Wikipedia entry is wrong.

If the Indicator fails, the signal can still display the relevant aspect.

BTW how short are the signal blocks if 5-block signaling is necessary?

" said:

Obviously short enough that minimum braking distance is required to be spread over 3 signal block sections. This, of course, would be entirely dependent on the local conditions - line speed, type of traffic, gradients. This is like asking "How long is a piece of string"?

Well how long in feet...or if you must...meters.

" said:

If the Indicator fails, the signal can still display the relevant aspect.

If The junction repeater fails the signal that it is on will drop down to a caution. This is regardless of the aspect of the next signal. That way the driver will take action accordingly and not assume the next signal is set for the non diverging route.

" said:

BTW how short are the signal blocks if 5-block signaling is necessary?

Some signal-to-signal blocks between Illawarra Junction and Strathfield on the main west line are not much more than an eight-car train length plus overlap (approx 250-300 metres).

Track speed in this area is between 60 and 100 kph.

Signal blocks on the City Underground and Eastern Suburbs Railway are as close as 74 metres. Signal spacing here is styled on the London Underground "Multi-Home" strategy. Track speed is 50 to 60km/h.

On the other hand, looking out towards St Mary's - Penrith way, signals are up to 1,500 metres apart with a line speed of 115km/h and 4,000 tonne freighters up to 1,400m long mixing it up with MU commuter, inter-regional and interstate passenger traffic.