Issues mixing 3 and 4 aspect signalling?

[pompeyfan]

While in a day dream, I had a thought. So a driver is trundling along nicely at 60mph and passes a green proceed aspect on a 3 aspect signal. The next signal is a 4 aspect signal showing a single yellow? Is there an issue with that? Is that how it usually works?

 

[Eccles1983]

Happens in lots of places.

It's all apart of route knowledge to understand where this can happen. It shouldn't make a difference really if you realise it's a single yellow. The signalling distances should allow the train to safely stop within the distances between the caution and stop signal.

 

[Llama]

Yes, that's how it works.

Drivers will be aware of the transition of aspects and there should always be service braking distance from the first restrictive aspect to the red aspect. There are a few places where as it happens the most commonly encountered aspect sequence is YY after a three aspect G. It can then be a slight risk for a driver to become so accustomed to that sequence that they relax their concentration in that area, subconsciously revert to previous experience and perhaps don't register the fact that the first four aspect signal might be Y instead of YY.

Obviously this can also be an issue with frequently encountered aspect sequences in other areas where there is no 3-4 or 4-3 aspect transition and the restrictive sequence is one signal earlier than usually encountered.

What often happens in current resignalling schemes where there is a 3-4 aspect transition is that the next signal after the first four aspect signal becomes a distant signal (unable to display a danger aspect) which removes the issue, although it was never really an issue in the first place.

Such signals are often referred to as 'four aspect distant' signals even though they can only normally display three aspects (G, YY, Y) - because a driver is in a four aspect signalling area and should receive the normal four aspect restrictive signalling sequence to a signal at danger in advance, of G-YY-Y-R.

Interestingly, with a 4-3 aspect signalling transition where the first 3-aspect signal is on the route after taking a diverging junction, the sequence that a driver would receive if the second signal after the diverging junction is at danger, is:
G at the four aspect junction signal, then Y at the first signal after the junction on the diverging route, then R. Effectively you miss out the YY aspect.

 

[pompeyfan]

Thank you very much. Quick and concise replies.

 

[MarkyT]

Another example is in predominantly 2-aspect country, where the first distant approaching a station can display either a single or a double yellow for a red at the outer or inner homes respectively. Clearly there's sufficient braking distance from line speed at yellow to stop at the first home signal. The double yellow is by definition over-braked to the next home but as long as it's not excessive it is usually acceptable, at least to retain in older layouts until next resignalling.

 

[ForTheLoveOf]

I've often wondered why some lines, whose signalling appears to be of uniform vintage, have seemingly random variations of three and four aspect signalling. For instance, between Bristol Parkway and Gloucester. Is it just a historical quirk? Are all such areas now planned to have four aspect signalling when or if they are next resignalled (which must surely be coming up at some point, for that example)?

 

[Mag_seven]

I've often wondered why some lines, whose signalling appears to be of uniform vintage, have seemingly random variations of three and four aspect signalling. For instance, between Bristol Parkway and Gloucester. Is it just a historical quirk? Are all such areas now planned to have four aspect signalling when or if they are next resignalled (which must surely be coming up at some point, for that example)?

Traffic frequency and line speed has a lot to do with it.

 

[R G NOW.]

While in a day dream, I had a thought. So a driver is trundling along nicely at 60mph and passes a green proceed aspect on a 3 aspect signal. The next signal is a 4 aspect signal showing a single yellow? Is there an issue with that? Is that how it usually works?

Yes I have this very situation in Gloucester. A train from Cardiff comes up line towards G29(3 aspect) and gets a green, then signal G31(4 aspect) is off showing a single yellow and his train would move towards G35 which would be red as the barriers could still be up. Signal G31 is a four aspect one with position 1,2,4 AND 5 junction indicators, plus a draw ahead and little stencil one, when a route is set to the routes that require a junction indicator, the routes are approach controlled so signal G29 will show a single yellow, then G31 shows a position 2 junction indicator followed by a single yellow. (routed to platform 4.) Which is used the most. Although the position 5 junction also is used when the Fishguard harbour train terminates in platform 3.

 

[MarkyT]

Traffic frequency and line speed has a lot to do with it.

And a bit of localised four aspect around stations and junctions can allow a little 'closing up' without any weird approach controlled aspect sequences. This allows a following train to get closer to an occupied platform ahead when it catches up with the train in front loading there; keeps the second train rolling forward slowly rather than having to hold it a mile or more back, so hopefully it won't have to make an unexpected stop. Elsewhere three aspect spacing is perfectly adequate for desired separation between moving trains so the extra costs of four aspect throughout, i.e. more signals and electrical complexity, are avoided. More modern signalling schemes tend not to switch about back and forth so much between 3 and 4 aspect sequences, preferring a defined point along the line where the change takes place once. Note four aspect sequence islands with intermediate long block sections controlled by two aspect signals are perfectly feasible. That wouldn't count as a switch to three aspect rules as long as each island fully complied with the four aspect distant sequence rules.

 

[R G NOW.]

Some on the Filton bank were changed to 4 aspect.

 

[DanDaDriver]

Try some colour lights mixed with AB semaphores for the full mind-blowing experience.

 

[bengley]

You only really need to know when you can get a single yellow after a green in these types of situations so you don't look like a numpty reporting an irregularity to the signaller.

One thing you can guarantee is that if a single yellow is displayed (correctly) you will be able to stop before the red.

 

[Ken H]

Try some colour lights mixed with AB semaphores for the full mind-blowing experience.

Settle Carlisle is like that. Lots of intermediate block signals and colour light distants, but semaphore home ans starters.

and an odd 3 aspect one southbound south of Settle Jct

 

[Eccles1983]

Try some colour lights mixed with AB semaphores for the full mind-blowing experience.

Try colour light distant, semaphore distant, non block stop semaphore, home semaphore.

I give you Parbold on the down.....

 

[DanDaDriver]

Try colour light distant, semaphore distant, non block stop semaphore, home semaphore.

I give you Parbold on the down.....

Just do 20 at every magnet and you’ll be fine.

 

[Tomnick]

Try colour light distant, semaphore distant, non block stop semaphore, home semaphore.

I give you Parbold on the down.....

How about...?

Colour light distant (Y/G)
Semaphore home signal / IB outer distant
Semaphore section signal / IB inner distant
Colour light IB home / gatebox distant
Colour light gate box stop signal (non block)

 

[driver_m]

Even happens on the West Coast Mainline. Going through Stone towards Hixon, (South of Stoke on Trent) you can get a green just before the junction and the 3 aspect after that showing a single yellow. (Usually because the level crossing hasn't been closed)

 

[MichaelAMW]

Yes, that's how it works.
What often happens in current resignalling schemes where there is a 3-4 aspect transition is that the next signal after the first four aspect signal becomes a distant signal (unable to display a danger aspect) which removes the issue, although it was never really an issue in the first place.

So, have I got this right: that 4-aspect signal (I see why you call it that) actually only has 3 aspects, i.e. R, YY and G?

 

[pompeyfan]

So, have I got this right: that 4-aspect signal (I see why you call it that) actually only has 3 aspects, i.e. R, YY and G?

No, the first 4 aspect signal is proceeded by a 4 aspect distant which can show G, YY or Y, so that if the first true 4 aspect signal is only showing a Y, the driver still sees the YY on the distant.

 

[MichaelAMW]

No, the first 4 aspect signal is proceeded by a 4 aspect distant which can show G, YY or Y, so that if the first true 4 aspect signal is only showing a Y, the driver still sees the YY on the distant.

No, Llama said: "...the next signal after the first four aspect signal becomes a distant signal..." and I'm talking about the four-aspect signal he's referring to in that sentence. As I see it, the signal aspects will be:

3 aspect - R, Y, G
3 aspect - R, Y, G
3 aspect - R, Y, G
4 aspect, sort of - R, YY, G
4 aspect distant - Y, YY, G
4 aspect - R, Y, YY, G
4 aspect - R, Y, YY, G

Or, perhaps, with your use of the term "first true 4 aspect signal" we're actually in agreement?!

 

[ChiefPlanner]

Roughly speaking - 4 aspect for intensive areas , 3 aspect sort of equates to the older mechanical system - but modernised. Lower the aspects , the lower the speed and capacity.

Think LUL with (pre ATO) , 2 aspect signals but with repeaters - so you get green over yellow (proceed , but with restriction ahead , or green over green - "line clear" , - BUT your maximum speed is not much more than 45 -50 anyway)

Very complicated on routes which have had partial resignalling etc - or amendments - I recall a "row" over resignalling of the Basingstoke to Southcote Jucntion via Bramley- where the pedants insisted on "like for like" - i e 2 aspect - replicating a basic colour light replacement of an old colour light replacement. 3 aspect allowed the likes of class 4 Freightliners to run with much more confidence than before - in simple terms reducing potential SPAD risk and easing the flow of traffic on an increasingly busy route.

 

[Llama]

The Group Standards seem to define a four aspect signal as one where the driver sees the normal four-aspect sequence of G-YY-Y-R, whereas in most modern transitions from 3-4 aspect signalling where the distant is used the first stop signal and the distant signal as I described it above can physically only show three different aspects - G/YY/R at the first stop signal and G/YY/Y at the distant. I would still class them as four aspect signals.

 

[big all]

While in a day dream, I had a thought. So a driver is trundling along nicely at 60mph and passes a green proceed aspect on a 3 aspect signal. The next signal is a 4 aspect signal showing a single yellow? Is there an issue with that? Is that how it usually works?

i thought about this quickly and thought that wont happen then thought off course it can
as the distance from the four aspect [ B ]will be far enough to give braking distance to the next signal being at red[c] but the one in advance [d]would be too close to give a single yellow from the preceding signal requiring the need for four aspect
so the three aspect [a]

so signal D at red A green B 2 yellow C 1 yellow D red
signal C at red A green B 1 yellow C red

sorry about the heavy print dont know what happened there ??

 

[MarkyT]

No, Llama said: "...the next signal after the first four aspect signal becomes a distant signal..." and I'm talking about the four-aspect signal he's referring to in that sentence. As I see it, the signal aspects will be:

3 aspect - R, Y, G
3 aspect - R, Y, G
3 aspect - R, Y, G
4 aspect, sort of - R, YY, G
4 aspect distant - Y, YY, G
4 aspect - R, Y, YY, G
4 aspect - R, Y, YY, G

Or, perhaps, with your use of the term "first true 4 aspect signal" we're actually in agreement?!

The 'sort of' signal is where the 4-aspect sequence starts, even though it can only show three distinct aspects itself.
Braking distances (SBD) vary as follows (minimum):

3 aspect - R, Y, G
(1 x SBD)
3 aspect - R, Y, G
(1 x SBD)
3 aspect - R, Y, G
(1 x SBD)
4 aspect, sort of - R, YY, G
(0.5 x SBD)
4 aspect distant - Y, YY, G
(0.5 x SBD)
4 aspect - R, Y, YY, G
(0.5 x SBD)
4 aspect - R, Y, YY, G
etc.

 

[R G NOW.]

Has no one yet seen a 4 aspect signal that has a red R single yellow Y and a double yellow YY, but no green as this is apparently blanked out with a plate over it. (G335)

Note to bigall I think you must of selected bold somewhere?.

 

[MarkyT]

I recall a "row" over resignalling of the Basingstoke to Southcote Jucntion via Bramley- where the pedants insisted on "like for like" - i e 2 aspect - replicating a basic colour light replacement of an old colour light replacement. 3 aspect allowed the likes of class 4 Freightliners to run with much more confidence than before - in simple terms reducing potential SPAD risk and easing the flow of traffic on an increasingly busy route.

I recall that when I was at Waterloo. I believe BARS went for continuous 3-aspect in the end. The braking distance was a bit tight on the old signalling too, especially for very heavily loaded class 4s.

 

[MarkyT]

Has no one yet seen a 4 aspect signal that has a red R single yellow Y and a double yellow YY, but no green as this is apparently blanked out with a plate over it. (G335)

Outer home approaching a terminus would be one possible case.

 

[R G NOW.]

Outer home approaching a terminus would be one possible case.

No, this one is funny, it is actually before another signal which is two aspect (G362) and leads to another one which is 4 aspect (G364) with a position 1 junction indicator. it is approach controlled and clears to whatever the line status is after it at Tuffley junction.
There is no further travel and so the signal will always show the junction indicator.

 

[DanDaDriver]

.... in simple terms reducing potential SPAD risk and easing the flow of traffic on an increasingly busy route.

Good point. Whilst you may have sufficient braking distance from the single yellow to the red, if the only warning you get is the single yellow then you can have quite a while to forget about the red in a long section. Particularly if you’ve been following yellows for some time.

 

[Llama]

Has no one yet seen a 4 aspect signal that has a red R single yellow Y and a double yellow YY, but no green as this is apparently blanked out with a plate over it. (G335)

There was one at Bolton on the Down Passenger Loop but that went a couple of years ago with the resignalling, there must be others about.