Two aspect colour light signal question.

[NorthEndCab]

On 07/03/2020 at 19:02, Pandora said:

Wierd arrangement to have a Green before the buffer stops,   when  all you need the signal to have is the  Yellow lens  in front of the bulb. I don't suppose it would bother the  Old Guard on the Footplate too much

I’m guessing this was the solution to speeding into termini before the use of TPWS?

I’m only a humble boil-in-the-bag, and my instructor told me to regard red on the terminus blocks as a fixed red signal, hence always being checked down to it.
 

 

[The Stationmaster]

2 minutes ago, NorthEndCab said:

I’m guessing this was the solution to speeding into termini before the use of TPWS?

I’m only a humble boil-in-the-bag, and my instructor told me to regard red on the terminus blocks as a fixed red signal, hence always being checked down to it.
 

 

The final solution of the single yellow reading to an  to an unoccupied terminal platform was no doubt in many respects down to the fact that it would ensure a Driver got an AWS siren, and 'sunflower' instead of a bell as a very positive reminder that he was coming to the end of the line.  There is some logic too in regarding it as a single yellow reading to a fixed red at the stop blocks but there was no requirement to prove the red at the time the change was made although it is not a bad way to remember what you are doing.

 

As you will have found reading through this thread the idea about what aspect a Driver should receive from a colour light signal when approaching a terminus had been the subject of considerable variation during the preceding 30+ years.  in the light of Moorgate the change to single yellow undoubtedly made a lot of sense.  Semaphore signals of course still give a green light at night when approaching a terminal platform (although there probably aren't very many of them still around in 2020).

[NorthEndCab]

23 minutes ago, The Stationmaster said:

Semaphore signals of course still give a green light at night when approaching a terminal platform (although there probably aren't very many of them still around in 2020).

Skegness is the only one I can think of off the top of my head, but it’s been some time since I was in the front end there and it may have been resignalled. 

[ElectroSoldier]

Buxton has a green aspect for station approach.

 

There are cases where a 2 aspect colour light signal can only show a yellow aspect, for instance where the route set can only be to a diverging route.

[The Stationmaster]

16 hours ago, NorthEndCab said:

Skegness is the only one I can think of off the top of my head, but it’s been some time since I was in the front end there and it may have been resignalled. 

I think it might have been resignalled - a lot of the semaphores at Skegness went a few years back.

[jim.snowdon]

On 12 March 2020 at 23:43, The Stationmaster said:

Semaphore signals of course still give a green light at night when approaching a terminal platform (although there probably aren't very many of them still around in 2020).

Although presumably preceded by a (fixed) yellow, indicating that any of the subsequent stop signals may showing red, including the lamp on the buffer stop.

 

Jim

[The Stationmaster]

12 hours ago, jim.snowdon said:

Although presumably preceded by a (fixed) yellow, indicating that any of the subsequent stop signals may showing red, including the lamp on the buffer stop.

 

Jim

I presume that will inevitably be the case for new work nowadays Jim (using a retro reflective distant board instead of a signal.  But in the not too distant past there were still variations in Company/Regional practice that could be found and some companies did go in for worked distants approaching termini while others did not and very often Pre-Group practice continued not only into the Grouped period but also into BR Regional variations.

[Nick C]

On 07/03/2020 at 22:41, Nearholmer said:

One possible difference between semaphore practice and TCB colour-light is that signalmen would hold the home 'on' until they could see that a train was well under control, before clearing it to allow that train to run towards the stops at a terminus or towards an 'on' starter signal at a station. I've even seen a signalman 'waggle' a stop semaphore before pulling it off properly, as a way of alerting a driver to the fact the next signal, an advance starter, was still 'on'.

 

The rule is (exact wording may vary):

 

Quote

If you cannot clear a stop signal, for whatever reason, you must not clear the preceding stop signal until the train is at, or nearly at, a stand.

 

The implementation will obviously vary from box to box, but usually involves some combination of track circuit occupation or treadle sounding to alert the signalman as to where the train actually is...

[Nearholmer]

Which leaves ambiguity in the case of a terminus, because it depends upon whether or not the buffer-stop lamp is viewed for these purposes as a stop signal which can never be cleared.

 

Since writing what I wrote, I’ve wondered about the practicality of using the home to ‘control down’ trains approaching a very busy terminus, as opposed to a quiet place where a signalman would likely only be dealing with one train at a time. 
 

To do so would put one heck of a load on the signalman, even in a multi-staffed box.

 

What was actually done at places like busy London Termini and busy seaside resorts, where trains arrived ‘thick and fast’?

[Mark Saunders]

52 minutes ago, Nearholmer said:

Which leaves ambiguity in the case of a terminus, because it depends upon whether or not the buffer-stop lamp is viewed for these purposes as a stop signal which can never be cleared.

 

Since writing what I wrote, I’ve wondered about the practicality of using the home to ‘control down’ trains approaching a very busy terminus, as opposed to a quiet place where a signalman would likely only be dealing with one train at a time. 
 

To do so would put one heck of a load on the signalman, even in a multi-staffed box.

 

What was actually done at places like busy London Termini and busy seaside resorts, where trains arrived ‘thick and fast’?

 

The Rule Book and Signalmans General Instructions do not differentiate between busy and quiet locations! 

 

There is no ambiguity that the lamp on the buffers is the same as a stop signal.

 

The controlling of trains using a stop signal by bringing a train to a stand or nearly to a stand is normal with semaphore signals.

[Nearholmer]

56 minutes ago, Mark Saunders said:

The Rule Book and Signalmans General Instructions do not differentiate between busy and quiet locations! 

Which doesn’t surprise me, but neither does it answer my question about what was actually done.

 

TBH I’m struggling to envisage operating a busy terminus in a snappy fashion with so much signalmans’ intervention in checking of trains to ‘walking pace’. Broad Street? Liverpool Street?

 

I wonder if moving film exists to show how the signals were actually worked?

[bécasse]

5 minutes ago, Nearholmer said:

 

 

TBH I’m struggling to envisage operating a busy terminus in a snappy fashion with so much signalmans’ intervention in checking of trains to ‘walking pace’. Broad Street? Liverpool Street?

Until probably the mid-1960s there was no checking of trains at the approach to a terminus even with such (few) colour light signalling schemes as existed, although position light calling on signals would have been approached controlled from (probably) the late 1950s (furthermore the yellow running light equivalents on the Southern certainly weren't approach-controlled, the last such new installation being at Cannon Street in 1958). Drivers knew that they had to slow down approaching a terminus, it was part of their route knowledge - and their professional approach to the job.

[Nearholmer]

That rather confirms my gut-feel about how things must have worked to get the through-puts that were achieved in practice.

 

 

Edited March 16, 2020 by Nearholmer

[The Stationmaster]

19 hours ago, Nearholmer said:

Which doesn’t surprise me, but neither does it answer my question about what was actually done.

 

TBH I’m struggling to envisage operating a busy terminus in a snappy fashion with so much signalmans’ intervention in checking of trains to ‘walking pace’. Broad Street? Liverpool Street?

 

I wonder if moving film exists to show how the signals were actually worked?

We are in some respects talking about 'then' and 'now'.  Rule 39(a) was very specific that a stop signal must not be lowered until a train has been brought nearly to a stand at it if the stop signal immediately in advance of it and worked by the same signal box could not be lowered (didn't apply in respect of IB Homes or multiple aspect signals before anyone jumps in).  And that has carried through to today albeit with trimmed wording.  

 

BUT the idea that a stop block red light counts as the equivalent of a running signal is a modern concept and I suspect might only apply when colour light signalling is involved - SE might be able to tell us about any controls he was aware of? All this meant in the past was that so as long as the platform line was empty a Signalman would pull off for arriving trains at, usually, the earliest opportunity.  One reason for doing that would be that if there were any additional Home signals in rear of the one controlling entrance to the platform they too could be pulled off at the earliest opportunity and no delays would result.  And that, I trust, answers 'Nearholmer's'  concerns?

 

There were instances where Rule 39 was modified (in the form of some sort of relaxation) in the Signal box Special instructions for specific local reasons - particularly steep or awkward rising gradients.  None of this changed. except in terms of the Rule reference, when the 1972 Rule Book was issued.

 

Incidentally as far as WR colour light signalling was concerned the Regional standard of a single yellow on a colour light signal approaching a terminus definitely involved no controls associated with lights at the stop blocks of a terminus (especially as some of those lights were oil lamps!) and I'm pretty sure it was exactly the same when the Region was brought into line with the BR standard (of a green aspect) later in the 1960s)

[Nearholmer]

 

Alles ist klar.

 

As I thought, given the ‘density’ of the timetables in old Bradshaws.

 

Similar evolution of thinking has clearly occurred wrt clearing points, because some of the services operated on e.g. The District Railway, could only work if the distance between the tail of one train and the nose of another was allowed to be as little as ‘the thickness of a stick’.

 

K

Edited March 17, 2020 by Nearholmer

[The Stationmaster]

2 hours ago, Nearholmer said:

 

Alles ist klar.

 

As I thought, given the ‘density’ of the timetables in old Bradshaws.

 

Similar evolution of thinking has clearly occurred wrt clearing points, because some of the services operated on e.g. The District Railway, could only work if the distance between the tail of one train and the nose of another was allowed to be as little as ‘the thickness of a stick’.

 

K

Spot on yet again.  I don't know about the District railway (but that sounds right enough) but there were definitely overlaps on the South Eastern division of the SR which a past Signalling Design Engineer of my acquaintance described as 'the thickness of a signal post'.   And having on occasion crossed our offices to look out on the South Eastern side (instead my normal view of the International platforms) the headways to & from Charing Cross went a long way to bearing out his remarks - although officially the shortest permitted overlap for many years was actually 25 yards.

[phil-b259]

56 minutes ago, The Stationmaster said:

Spot on yet again.  I don't know about the District railway (but that sounds right enough) but there were definitely overlaps on the South Eastern division of the SR which a past Signalling Design Engineer of my acquaintance described as 'the thickness of a signal post'.   And having on occasion crossed our offices to look out on the South Eastern side (instead my normal view of the International platforms) the headways to & from Charing Cross went a long way to bearing out his remarks - although officially the shortest permitted overlap for many years was actually 25 yards.

 

It was once said to me that the units of measurement were deliberately missing from some diagrams. Thus a '25 Overlap' could well be read as 25 yards by the unknowing rather than the 25 inches it really was!

 

I also believe that such non existent overlaps were got rid of in the 1976 resignalling of London Bridge which saw the abolition of sorting out Charing Cross and Cannon Street trains at Borough Market junction with said 'sorting out' happening further east on the approach viaducts around the Spa Road area.

Edited March 17, 2020 by phil-b259

[TheSignalEngineer]

9 hours ago, The Stationmaster said:

BUT the idea that a stop block red light counts as the equivalent of a running signal is a modern concept and I suspect might only apply when colour light signalling is involved - SE might be able to tell us about any controls he was aware of? 

One terminus I was familiar with was Euston. When commissioned that had a run of signals RYGY, RYG and RG on the approach. This means that it was possible to run in all the way down Camden Bank on greens into an empty platform with no restriction. I have the box instructions book somewhere so I will come back if there is anything different.

[TheSignalEngineer]

Before I altered it post-Moorgate, Birmingham Moor Street had a straight run in on greens. After Moorgate it was changed to have a G/YY/Y sequence into an empty platform without any approach controls. You knew when arriving at Small Heath if you had a clear run into Moor Street.

[5BarVT]

Buffer stop lamps, even if electric weren’t proved in WR signals when I was there. (Different department - E&P did them, usually mains bulkhead lamps.).  It is only very recently that braking distance from home signal to buffers has become a requirement. I regret to say that it might have been introduced by an engineer who thought it had always been the case due to lack of old hand knowledge.  That then leads to undesirable hazards when the home signal at a terminus always clears on approach.  Unfortunately a rule that is not ALARP in my opinion.

Paul.

[TheSignalEngineer]

4 minutes ago, 5BarVT said:

Buffer stop lamps, even if electric weren’t proved in WR signals when I was there. (Different department - E&P did them, usually mains bulkhead lamps.).  It is only very recently that braking distance from home signal to buffers has become a requirement. I regret to say that it might have been introduced by an engineer who thought it had always been the case due to lack of old hand knowledge.  That then leads to undesirable hazards when the home signal at a terminus always clears on approach.  Unfortunately a rule that is not ALARP in my opinion.

Paul.

More usual to have a blanket speed restriction through all lines in the station throat, then the speed at the Home signal if overlap clear of all points would be low enough to give BD to the stop block. Sounds like you have someone who thinks that if a unit can do 125mph then the last signal or double yellow reading up to it should give 125mph BD to the stop block.

 

Moor Street had 60mph coming in from Tyseley South, a 200 yard overlap at the Home signal then beyond that 15mph through the pointwork and platforms capable of taking at least 9 coaches. It had oil lamps on the stop blacks and in 12 years commuting into there we never hit them once.

[beast66606]

45 minutes ago, TheSignalEngineer said:

Before I altered it post-Moorgate, Birmingham Moor Street had a straight run in on greens

 

Liverpool Lime Street too

[jim.snowdon]

6 hours ago, The Stationmaster said:

Spot on yet again.  I don't know about the District railway (but that sounds right enough) but there were definitely overlaps on the South Eastern division of the SR which a past Signalling Design Engineer of my acquaintance described as 'the thickness of a signal post'.   And having on occasion crossed our offices to look out on the South Eastern side (instead my normal view of the International platforms) the headways to & from Charing Cross went a long way to bearing out his remarks - although officially the shortest permitted overlap for many years was actually 25 yards.

The London Underground railways had one distinct advantage when it came to engineering minimal overlaps - trainstops. Pass a signal showing a stop aspect and the train brakes were applied. No argument, and nothing left to the judgement of the driver. The stopping distances were calculated, rather conservatively, by working out what speed the train would arrive at the signal concerned on the basis of nil passenger load and maximum acceleration, the braking distance from that speed assuming maximum load and one car's worth of brakes not operating.

 

Taking Baker Street Outer Circle as an example, there is a converging junction not far off the end of the platfrom, which fixes the limit of the overlap. The position of the protecting signal is fixed by the end of the platform, so, by working backwards, the maximum speed of a train approaching that signal can be calculated. It is, not surprisingly, rather lower than the line speed. To get round this, another signal is placed further back up the platform at the point where, if the train is tripped from normal speed, it will still stop short of the conflict point. As the train runs in, this signal shows a red aspect until under approach control, it is released to show a yellow aspect and its trainstop lowered. The calculations assume that if the train then accelerated towards the red at the end of the platform, the speed at which it would strike the trainstop of the platform end signal would still be low enough for it to come to a stand short of the conflict point. If the junction protecting signal was showing a clear (green) aspect, the mid-platform signal would also show green (thus making them the only true three aspect signals on the Underground).

 

Similar situations existed at Edgware Road (Met) and at Aldgate, the complications of which, I believe, were too much for the first contractor tasked with the conversion of the Sub-Surface LInes signalling to CBTC.

 

Jim

[Nearholmer]

12 minutes ago, jim.snowdon said:

The London Underground railways had one distinct advantage when it came to engineering minimal overlaps - trainstops.

 

When were train-stops introduced?

 

I thought they came from the US c1900, with the US-style signalling, whereas when I said "District Railway" I was thinking of the services operated under manual semaphore prior to that.

 

I've got an old text book with timetable graphs in it, and the headways were very tight indeed, and the District became famous for 'service perturbations' (snarl-ups) due to timetabling more trains than the signalling at the time could sensibly support, so 'nose-to-tail' must have been a very common situation. There were definitely several accidents, I think all slow-speed ones, caused by drivers over-running signals in the subterranean smog and hitting a train only a few yards beyond the signal.

 

Interesting that you mention 're-motoring' risk being factored-in when calculating overlaps on LU; is it on main lines?

 

 

Edited March 17, 2020 by Nearholmer

[Grovenor]

Quote

Interesting that you mention 're-motoring' risk being factored-in when calculating overlaps on LU; is it on main lines?

Overlaps have to be calculated on LT and similar signalling systems because the overlap is required to cover the worst case braking distance from a trip.

On the main line (UK) signalling the overlap is usually well under braking distance and is provided as some mitigation for drivers misjudgement, so there is no way to calculate it and standard overlap lengths are chosen.

Block signalling traditionally used 440 yds. Track circuit block with colour lights used 300 yds for 3 aspect and 200 yds for 4 aspect as it was considered that provision of two warnings reduced the likely error. And always with the possibility of reduction for special circumstances.

Edited March 17, 2020 by Grovenor