Signal spacing and where 4 aspects are needed

[GC class B1]

Moderator note: Split from https://www.railforums.co.uk/thread...ar-salisbury-fisherton-tunnel-31-10-21.224217

In a way it does though - otherwise we'd just have red and green. Of course that's the actual reason but in reference to the comment it has a side effect of increasing separation because the driver won't be going at line speed the whole time under cautionary aspects.

I am not a signalling specialist but my understanding is that four aspect signalling is more a requirement because of the higher line speed and therefore longer stopping distances.

 

[hexagon789]

I am not a signalling specialist but my understanding is that four aspect signalling is more a requirement because of the higher line speed and therefore longer stopping distances.

125mph is allowed on 3-aspect
and 110 on 2-aspect that I'm aware of.

I understood 4-aspect permitted shorter headways by reducing the space between signals and thus increased capacity rather than enabling higher speeds per se.

 

[Efini92]

125mph is allowed on 3-aspect
and 110 on 2-aspect that I'm aware of.

I understood 4-aspect permitted shorter headways by reducing the space between signals and thus increased capacity rather than enabling higher speeds per se.

The gwr mainline has 125mph on 3 aspects in part. It was a wake up the first time i saw a yellow at 125.

I believe 4 aspect signalling was introduced as the speeds got higher. But you’re right, a train can travel faster running on 2 yellows than it can on one yellow.

 

[hexagon789]

The gwr mainline has 125mph on 3 aspects in part. It was a wake up the first time i saw a yellow at 125.

I believe 4 aspect signalling was introduced as the speeds got higher. But you’re right, a train can travel faster running on 2 yellows than it can on one yellow.

And there is definitely 125 on three aspects north of Newcastle on the ECML, and 110 on two-aspect. I don't think there's 125 on two-aspect but I'll happily stand corrected on that.

4-aspect using a double yellow dates from the 1920s, so it's not a product of the higher speeds, I believe the first installation was on the southern and was to add capacity on Holborn Viaduct by reducing headways.

 

[GC class B1]

The gwr mainline has 125mph on 3 aspects in part. It was a wake up the first time i saw a yellow at 125.

I believe 4 aspect signalling was introduced as the speeds got higher. But you’re right, a train can travel faster running on 2 yellows than it can on one yellow.

Am I right then in that with 3 aspect signalling in 125 MPH areas headways will need to be longer as signals will need to be further apart to allow for longer stopping distances.

 

[brad465]

125mph is allowed on 3-aspect
and 110 on 2-aspect that I'm aware of.

I understood 4-aspect permitted shorter headways by reducing the space between signals and thus increased capacity rather than enabling higher speeds per se.

IIRC a 2 aspect 125mph section exists through Box tunnel on the GWML, presumably given both the tunnel's length and desire not to have a train stop in it.

 

[Signal Head]

The gwr mainline has 125mph on 3 aspects in part. It was a wake up the first time i saw a yellow at 125.

I believe 4 aspect signalling was introduced as the speeds got higher. But you’re right, a train can travel faster running on 2 yellows than it can on one yellow.

Not so, as you say yourself the GWML has 125 running on 3 aspects.

What 4 aspect does, is increase or preserve capacity at higher line speeds by allowing signals to be closer together, so 'space' (length of track) is not wasted simply to provide sufficient braking distance.

As an example, if a stretch of line is signalled with 3 aspects, at a line speed of (say) 75MPH, then if you want to raise the line speed (let's say to 125), you could space the signals further apart, but that costs line capacity as the blocks are longer, so less of the line is occupied by vehicles.

Alternatively, add a top Y aspect to all your signals, now you get double the braking distance, but with signals at the same spacing, speed increases without sacrificing capacity.

That's a very simplistic illustration, in practice you'd probably have to respace the signals as well, because you probably don't need double the BD, so leaving them in the same positions would result in 'overbraked' signals, which can present their own problems.

 

[hexagon789]

IIRC a 2 aspect 125mph section exists through Box tunnel on the GWML, presumably given both the tunnel's length and desire not to have a train stop in it.

Thank you, obviously the possibility is there but I was only aware of 3-aspect with 125 and 110 with two.

Perhaps there is some 125 on the northern ECML on two-aspect then?

 

[Efini92]

Not so, as you say yourself the GWML has 125 running on 3 aspects.

What 4 aspect does, is increase or preserve capacity at higher line speeds by allowing signals to be closer together, so 'space' (length of track) is not wasted simply to provide sufficient braking distance.

As an example, if a stretch of line is signalled with 3 aspects, at a line speed of (say) 75MPH, then if you want to raise the line speed (let's say to 125), you could space the signals further apart, but that costs line capacity as the blocks are longer, so less of the line is occupied by vehicles.

Alternatively, add a top Y aspect to all your signals, now you get double the braking distance, but with signals at the same spacing, speed increases without sacrificing capacity.

That's a very simplistic illustration, in practice you'd probably have to respace the signals as well, because you probably don't need double the BD, so leaving them in the same positions would result in 'overbraked' signals, which can present their own problems.

Your description is a lot better than mine.

 

[norbitonflyer]

Am I right then in that with 3 aspect signalling in 125 MPH areas headways will need to be longer as signals will need to be further apart to allow for longer stopping distances.

Quite so - although there can be other solutions - part of the design brief of the Inter City 125 was better brakes, so that it cop0uld stop from125 intyhe distance a "Deltic+8" could stop from 100. This is also why you may sometimes see differential speed limits where speeds are lower - e.g 50/HST70, although there can be other reasons, such as lightweight DMUs being allowed faster speeds over certain bridges than heavier locos (e.g on the West Highland Line)

Not so, as you say yourself the GWML has 125 running on 3 aspects.

What 4 aspect does, is increase or preserve capacity at higher line speeds by allowing signals to be closer together, so 'space' (length of track) is not wasted simply to provide sufficient braking distance.

Four aspect may also be introduced without a change in maximum speed, to allow closer headways for slower trains. In this case, roughly speaking, the signal spacing would be halved, (this would obviously only be done as part of a major resignalling project) so that from full line speed a driver would still get the first restrictive aspect at the same distance before a red, but it would now be a double yellow, not a single. But with the signals closer together trains can get closes behind each other (at suitably cautious speeds). This can help move things along as when a section clears the following train can reach the next signal sooner.

 

[edwin_m]

With three aspect signalling, the signals have to be spaced at the braking distance of the train, plus various safety margins, so the train can stop from line speed between a single yellow and a red. With four aspect signalling they are spaced at half the braking distance, also plus various safety margins, so the train can stop from line speed between a double yellow and a red.

The technical headway is the time or distance between successive trains running on green signals. This is determined by the aspects of signals behind a preceding train when it is just about to clear the overlap of signal 1, so for three aspect they are as follows:
Signal 1: red
Signal 2: red (the train is in the overlap beyond signal 1)
Signal 3: yellow
Signal 4: green

So the headway is the distance from signal 1 to signal 4, plus the overlap length, sighting distance of signal 4 (often taken as the distance to signal 5) and train length. This is three braking distances plus extras.

With four-aspect signals they are as follows:
Signal 1: red
Signal 2: red (the train is in the overlap beyond signal 1)
Signal 3: yellow
Signal 4: double yellow
Signal 5: green

So the distance between trains is now the distance from signal 1 to signal 5, plus the same extras. But the signal spacing is now half the braking distance, so that's only two braking distances plus extras. So depending on various factors that define the extras, four aspect allows about 25% shorter headways.

 

[43066]

a train can travel faster running on 2 yellows than it can on one yellow.

We have some long 110mph three aspect sections. If you encounter restrictive aspects on these it really knackers you in terms of timings because a single yellow means slowing right down to 45mph or so. By contrast on our four aspect 125mph sections I would have no problem running at 80mph on YY snd 60mph on Y.

 

[Taunton]

And there is definitely 125 on three aspects north of Newcastle on the ECML, and 110 on two-aspect. I don't think there's 125 on two-aspect but I'll happily stand corrected on that.

If I remember correctly when 125mph running began in the mid 1970s it was 2-aspect from Didcot to Swindon, and possibly elsewhere, with signalling which had been installed in the late 1960s. The actual siting of distants and stop signals was pretty much equidistant along the section, and after some years they were changed to 3-aspect on the same posts. It would have a downside that, if following another train, you could be getting a succession of alternating yellow and green aspects.

 

[The Planner]

With three aspect signalling, the signals have to be spaced at the braking distance of the train, plus various safety margins, so the train can stop from line speed between a single yellow and a red. With four aspect signalling they are spaced at half the braking distance, also plus various safety margins, so the train can stop from line speed between a double yellow and a red.

The technical headway is the time or distance between successive trains running on green signals. This is determined by the aspects of signals behind a preceding train when it is just about to clear the overlap of signal 1, so for three aspect they are as follows:
Signal 1: red
Signal 2: red (the train is in the overlap beyond signal 1)
Signal 3: yellow
Signal 4: green

So the headway is the distance from signal 1 to signal 4, plus the overlap length, sighting distance of signal 4 (often taken as the distance to signal 5) and train length. This is three braking distances plus extras.

With four-aspect signals they are as follows:
Signal 1: red
Signal 2: red (the train is in the overlap beyond signal 1)
Signal 3: yellow
Signal 4: double yellow
Signal 5: green

So the distance between trains is now the distance from signal 1 to signal 5, plus the same extras. But the signal spacing is now half the braking distance, so that's only two braking distances plus extras. So depending on various factors that define the extras, four aspect allows about 25% shorter headways.

Pretty sure we don't calculate them like that. There is no assumption that the first train is in the overlap. Its overlap + train length + distance from red to green + sighting distance.

 

[MarkyT]

Pretty sure we don't calculate them like that. There is no assumption that the first train is in the overlap. Its overlap + train length + distance from red to green + sighting distance.

For 3 aspect, signals are nominally spaced at least at worst case braking Distance (D) for the traffic mix. Headway distance is then 2D + S + O + L (Sighting, Overlap, train Length). Headway time can then derived from the speed over the headway distance.
For 4 aspect, spacing is at least D/2 and headway is 1.5D + S + O + L
D varies in practice according to maximum and achievable speed for the different trains involved as well as the gradient, which are often all changing constantly. Average gradient between signals is used in the calculation. Here's some data extracted from the look-up table in the rail industry standard that defines the distances used (only level track figures shown).

Significant over-spacing or irregular spacing of signals is also avoided where possible as this can be difficult for drivers.

 

[edwin_m]

Pretty sure we don't calculate them like that. There is no assumption that the first train is in the overlap. Its overlap + train length + distance from red to green + sighting distance.

Thanks for clarification. I think that's what I meant, but I probably didn't explain it very well.

 

[John Webb]

It's worth noting that the 1920s introduction of four-aspect signalling was in part due to the very wide difference in braking capability that had arisen between different trains by then. Steam-hauled vacuum-braked stock did not have the braking force available that the electric multiple-unit stock that used Westinghouse or similar compressed-air operated brakes had. So a Y-Y aspect could be treated as a 'green' by the EMU stock but the vacuum-braked trains had to treat a Y-Y the same as a semaphore distant and start their braking that much sooner. Hence in particular the reason the Southern Railway with its extending electrification in the 1920s/30s rapidly adopted 4-aspect signalling on its busiest routes.