Two Aspect Signals

[JustinStacey.x]

Railway signalling fascinates me, but one thing that has always made me curious is why certain aspect signals are used.

I have a theory, but I don't know if it is correct...

From what I can tell, four aspect signals are quite often close together, which is understandable given they are quite informative.

Three aspect signals seem to be a little further apart, again understandable given they show only three lights.

But two aspect signals - why? I have seen a few of these on my local line and except for sidings and the like, I see them to be as much use as a chocolate fireguard. I sure hope they aren't placed consecutively one after another as a green on signal A means an automatic red on signal B and no way of properly knowing what's going on!

Can someone clarify these things?

 

[starrymarkb]

Often they are repeaters, or starting signals. Where it is expected a train will be near stationary for some time. They are also be used for long block sections backed up with repeaters on the appoach to the next signal

There are a few others aswell. I learnt from Simsig that the reason the signal on platfrom two at Exeter Central is two aspect is because the next signal must be green before it will release (given the gradient)

 

[JustinStacey.x]

Right, I was thinking so... bit like LuL signals then.

 

[me123]

They're also used when there is no option but only a red or a yellow. For example, approaching a terminus station, the last signal you see will either show red (do not proceed) or yellow (proceed with caution) because the next "signal" will be at red. Let's face it, there's no point have a green signal if it ain't ever going to be used!

As for two aspect signals, they generally have a repeater between them. For example, A123 and A124 may both be two aspect signals (a red and a green). However, in between them, there will be a "repeater" signal (A124R) with a green and a yellow. A green indicates the next signal (A124) is also green, a yellow indicates that the next signal is showing a red "danger" aspect.

 

[Dennis]

If you are interested in signalling take a look at signalbox.org

 

[Old Timer]

The type of signalling (two, three or four aspect) used is a function of the maximum speed of the line, distances between signals and the number of trains that it is required to run, and how close behind they will be (Headway).

This will then be the factor that determines which type is installed.

On some lines (the ECML for example), four aspect are installed on the Fast lines, and three aspect on the Slow lines sinply because that is what is required to operate the service on each line.

As a general rule two aspect signals replaced mechanical signals on a like for like basis.

In other cases, a line of route may be resignalled from new with two aspect (home and distant) signals, the Home displaying only red/green and the distant only displaying yellow/green. An example of this can be seen on the Main Line from Dublin to Cork.

 

[GB]

As for two aspect signals, they generally have a repeater between them. For example, A123 and A124 may both be two aspect signals (a red and a green). However, in between them, there will be a "repeater" signal (A124R) with a green and a yellow. A green indicates the next signal (A124) is also green, a yellow indicates that the next signal is showing a red "danger" aspect.

The new Network Rail standard for new signalling schemes appear to do away with the coventional repeater/distant signal identification numbers ( ie, D7R D7 D8R D8 D9R D9) in favour of unique nubers.

At least this is what has happened in the recent resignalling scheme between Colchester and Clacton.

 

[Old Timer]

That is probably because it is not a two-aspect AB replacement resignalling scheme.

I think you will find every signal is capable of displaying a red aspect and thus will be uniquely numbered on that basis.

I could be wrong of course but this is what I recall from looking at the Scheme Plan.

 

[Tomnick]

The new-ish signals at Appleby and Elsham (on the former MS&LR between Scunthorpe and Barnetby) are numbered uniquely, including the distants. They're almost like-for-like replacements (albeit now with full braking distance between home and starter) for the semaphore signalling, and absolute block is still in operation east from Appleby - so in the down direction you have EM3389 (Elsham's distant), EM3387 (Elsham's 3-aspect home), EM3385 (Elsham's 2-aspect starter), AY3375 (Appleby's distant), AY3373 (Appleby's 3-aspect home), and then you're into TCB territory and onto Scunthorpe's signals. The gap in the numbering sequence is interesting - it's a fairly long section, so I can only assume that 'spaces' have been left for additional signals in the future.

 

[GB]

That is probably because it is not a two-aspect AB replacement resignalling scheme.

I think you will find every signal is capable of displaying a red aspect and thus will be uniquely numbered on that basis.

I could be wrong of course but this is what I recall from looking at the Scheme Plan.

Nope, definitely a two aspect route with distants.

Example.

The signal protecting Alresford level crossing on the down is a two aspect stop signal plated as CO1115, its distant signal is CO1113.

 

[175001]

Its the reminiscent of old railways. Two Aspects have replaced Distant and Section semaphore signals on Absolute Block. The 2 Aspect Distant shows only Yellow and Green, whilst 2 Aspect Section signals show Red and Green only. The Home signal is a 3 aspect signal, which shows all the colors.

Lucky i've just done signals as part of my Conductor training course eh!

 

[Muttley]

.... The Home signal is a 3 aspect signal, which shows all the colors.....

As ever on the railway...not always.

Theres still a few relics that have red/green 2 aspect homes.



And its colours, we being east of the Atlantic.

 

[Tomnick]

Really? Not doubting you, but it's something that I've never seen, so it'd be interesting to learn more!

 

[hairyhandedfool]

Not all three aspect signals show a green either.

 

[SWT Driver]

Basingstoke's new signals are 2 aspect signals, yet they are capable of showing all 4 aspects.

R-Y-YY-G + (F)YY & (F)Y for trains on the down going to the WoE line at Worting Junction.

 

[Old Timer]

Nope, definitely a two aspect route with distants.

Example.

The signal protecting Alresford level crossing on the down is a two aspect stop signal plated as CO1115, its distant signal is CO1113.

I have had the Scheme Plan checked and whilst there are one or two conventional two aspect home/distant signals, the overall branch resignalling scheme is three aspect.

Currently the resignalling of the Clacton area from Burrs Road LC has been deferred indefinitely.

 

[O L Leigh]

Apologies, folks. Pedrantry insists I pick up a couple of points.

Not all three aspect signals show a green either.

Yes, we have a few signals like that. It is especially the case on approach to terminus stations where the last signal head will only be able to display 2 aspects (Y and R).

However, while the number of available aspects is not up to the full four, it is still 4 aspect signalling on the basis that the signals are just the last part of the 4 aspect sequence.

Basingstoke's new signals are 2 aspect signals, yet they are capable of showing all 4 aspects.

Ah, the much favoured LED. I would classify these as 2 lense/4 aspect signals.

But then, as I said, I'm a pedant.

O L Leigh

 

[WhereInSussex]

I'd go with what has already been said - if you replace a semaphore signal on AB with a colour light, then it will often be two aspect (R-G or R-Y) or a distant (Y-G). The spacing of the signals on the line and line speed are two factors in 3 or 4 aspect signalling on TCB mainly, also visibility due to buildings, bends etc.

An example of two aspect R-G signalling in a three aspect signal is at Newhaven Harbour, NH5, where the signal is either Red or Green when going down the branch (subject to a fixed distant further down towards Seaford IIRC) and the only time a yellow is shown is when they are going into the (dead end) Marine platform. An example of R-Y two aspect signalling is back a bit at Newhaven Town (CCO??) on the end of the down platform, which is (I think) the section signal and on the approach to a PSR and single line, so they can only show caution (at a guess)......

Of course, if anyone knows different, please correct me, as all of that is guess work!