Approach control signals on the Underground

[eldomtom2]

My understanding of approach control signals on National Rail is that drivers are not required to know which signals are approach controlled, and indeed there is a degree of encouragement not to do so as a driver may assume an approach controlled signal will clear when it won't.

However, from playing the Metropolitan Line in Train Simulator, approach control signals seem to work differently on LU, and I'm wondering if this is accurate or a limitation of the simulation. They appear to require the driver to drop below a specific speed by a certain point before the signal, and even if the driver stops short of the signal if they exceeded the prescribed speed over the aforementioned point before the signal the signal will not clear. The locations and prescribed speeds are not marked, thus necessitating drivers to know which signals are approach controlled and what speed they mandate. Is this accurate?

 

[Dstock7080]

That is indeed correct, stop short, too slow, too fast, won’t clear.

 

[eldomtom2]

That is indeed correct, stop short, too slow, too fast, won’t clear.

Is this just a historical leftover or are there specific reasons for keeping it this way?

 

[bramling]

My understanding of approach control signals on National Rail is that drivers are not required to know which signals are approach controlled, and indeed there is a degree of encouragement not to do so as a driver may assume an approach controlled signal will clear when it won't.

However, from playing the Metropolitan Line in Train Simulator, approach control signals seem to work differently on LU, and I'm wondering if this is accurate or a limitation of the simulation. They appear to require the driver to drop below a specific speed by a certain point before the signal, and even if the driver stops short of the signal if they exceeded the prescribed speed over the aforementioned point before the signal the signal will not clear. The locations and prescribed speeds are not marked, thus necessitating drivers to know which signals are approach controlled and what speed they mandate. Is this accurate?

There are all sorts of approach control on LU, or at least there were until much of it has been obliterated by the various line upgrade resignalling projects!

Some signals, or routes on a signal, will clear simply when a train reaches a particular point, regardless of speed.

In other cases there will be one or more timing sections.

So you might have a situation where a train has to occupy a track circuit for 15 seconds before the signal will clear.

Or it might be more complex, for example no less than 4.5 seconds through track circuit A, OR track circuit B occupied for 15 seconds. The speeds will depend on the physical length of the track circuits, but such a setup might mean the train either has to do (say) 20 mph through the first timing section, or if it exceeds this then it will have to come to a stand at the signal for it to clear.

There also used to be inductor rails where the train had to be doing a certain speed over the section of rail, and to complicate things even more these wouldn’t work if the train was too slow! My belief is all of these have been removed now, the Piccadilly Line was the last line to have these. By the end few actually worked due to lack of spare parts.

As to drivers knowing how things work, the answer is this varies. Drivers will generally know the obvious ones, and in some cases instructors will teach them. But there will be ones that drivers won’t know about, though the information is there if someone wants to look it up.

Nowadays the general philosophy is not to anticipate signals clearing, so there tends to be a leaning towards treating all signals the same. This is in contrast to the past when anticipating signals clearing was more common practice.

The bigger thing on LU is, unlike mainline, it is imperative that drivers draw well up to signals. In some cases stopping too far back will delay clearance, or may prevent clearance at all.

 

[LBMPSB]

There was one on the Westbound Cirle/Met/H&C platform at Moorgate I remember. there was a signal mid platform with a trip cock. It remaind red with a train approaching and then would clear after a predetermined time.

Drivers are not told of approach control signals on the National Network because of several accidents that have occurred where a driver has become complacent. Each time they approach a particular red signal it has always cleared up to a proceed at a particular point but on the one occasion it stays at red they have accelerated in anticipation of change to proceed, or not slow down as they should and result is an accident.

On the other hand a Driver not knowing a signal is approach control, is frustrating especially as most drive defensively and stop short of approach control controls on a signal and the signal never changes up and the signaller has to contact the driver. Some drivers even refuse to move towards a red signal from a stand until they are told they will be waiting there all day if they don't.

 

[Dstock7080]

There was one on the Westbound Cirle/Met/H&C platform at Moorgate I remember. there was a signal mid platform with a trip cock. It remaind red with a train approaching and then would clear after a predetermined time.

OE260 at Moorgate was a draw-up signal because of the converging junction ahead, not a speed controlled.
If the starting signal OE26 was clear, then OE260 was also clear.
If OE26 was at danger then the draw-up was initially also at danger, when the speed of the approaching train was checked the signal would display red and yellow. When the speed check was successful, the trainstop would lower and the red would extinguish, allowing the train up to the starter.

 

[LBMPSB]

OE260 at Moorgate was a draw-up signal because of the converging junction ahead, not a speed controlled.
If the starting signal OE26 was clear, then OE260 was also clear.
If OE26 was at danger then the draw-up was initially also at danger, when the speed of the approaching train was checked the signal would display red and yellow. When the speed check was successful, the trainstop would lower and the red would extinguish, allowing the train up to the starter.

Is Moorgate (Cirle/Met/H&C) now resignalled and in cab?

 

[Taunton]

I seem to recall similar on the Westbound District at Embankment, a mid-platform signal. Used to stand on the eastbound platform and see the trainstop lowering just as the westbounds approached.

This system used to be on the Toronto Subway (the TTC), multiple simple red/green signals with a timed approach to clear. Drivers got to know them completely and would proceed at the maximum allowed speed with them just changing, and the trainstop dropping. You could look out of the front of the train, and it looked extraordinary, approaching the red at speed, sometimes the signal only just changed to green fractionally before you flashed past it, and you wondered if the driver had even seen it change. I guessed that sometimes in the tunnel they could see the trip arm lowering. I wondered how many trips they got.

Was in use for many years but eventually they had a serious accident, two consecutive signals didn't clear because a train ahead was being delayed, not noticed, and the trainstop trip arm was out of adjustment.

 

[Dstock7080]

Is Moorgate (Cirle/Met/H&C) now resignalled and in cab?

Yes, CBTC since 10 January 2021

 

[Goldfish62]

OE260 at Moorgate was a draw-up signal because of the converging junction ahead, not a speed controlled.
If the starting signal OE26 was clear, then OE260 was also clear.
If OE26 was at danger then the draw-up was initially also at danger, when the speed of the approaching train was checked the signal would display red and yellow. When the speed check was successful, the trainstop would lower and the red would extinguish, allowing the train up to the starter.

I do find Underground signalling fascinating.

 

[etr221]

The principle - as I understand it - for LU Approach Controlled signals is that they are (or were) a distinct type of signal, essentially repeater signals (with trip stops) for a stop signal with a (sometimes very) short overlap (e.g. approaching the junctions at Baker Street (SSL)) - they are numbered based on the stop signal, with a zero added (e.g. MB140 was the AC repeater for MB14), and (unusually for LU) are three aspect: would be green if the stop signal was clear, red (with trip stop raised) if the stop signal was on, but would change to yellow (and lower trip stop) when a timing relay indicated that the approaching train was slow enough for the stop signal's trip stop to do its thing, and stop the train within the overlap if tripped.

 

[bramling]

The principle - as I understand it - for LU Approach Controlled signals is that they are (or were) a distinct type of signal, essentially repeater signals (with trip stops) for a stop signal with a (sometimes very) short overlap (e.g. approaching the junctions at Baker Street (SSL)) - they are numbered based on the stop signal, with a zero added (e.g. MB140 was the AC repeater for MB14), and (unusually for LU) are three aspect: would be green if the stop signal was clear, red (with trip stop raised) if the stop signal was on, but would change to yellow (and lower trip stop) when a timing relay indicated that the approaching train was slow enough for the stop signal's trip stop to do its thing, and stop the train within the overlap if tripped.

That’s the basic principle, however in practice *any* signal on LU can have a timing section on it if the designers deemed it necessary.

For example, on the Northern Line the approach to Stockwell northbound consisted of

A.668A - outer home
A.668B - intermediate home
U.11 - inner home
U.100 - in the platform
U.10 - starter

In the event of a train using the crossover, a northbound train would be held at U.11. Or the crossover could be used with a train at U.10, but only after any approaching train had been proved to have come to a stand in the platform.

In the event of a train standing in the reversing berth north of the crossover, signal U.11 would clear subject to EITHER a 15mph timing section, OR having come to a stand at the signal. Signal U.100 would also clear at 15 mph in this situation.

To complicate things even further, speed control *also* applied for northbound trains approaching with the northbound platform occupied. In this situation A.668A would clear subject to EITHER a 15 mph timing section OR coming to a stand at the signal. A.668B would also clear earlier subject to a timing section on the approach to the signal (plans don’t indicate the speed of this one).

So, altogether a total of SIX different timing sections, applying in their various ways to four different signals, to protect against two different situations - or more correctly to assist the flow of trains through by allowing trains to get closer than would otherwise be the case. Drivers wouldn’t be trained to this level of detail, though no doubt more observant ones might pick up how it all worked over time.

As an aside, signal U.100 had absolutely appalling sighting, being tucked under the platform edge, barely visible at all. Thankfully due to the aforementioned arrangements it should never have been approached at danger without having been checked at U.11 first, though presumably certain failure situations could result in an unplanned sequence of aspects. In that situation a SPAD investigation would have proved very interesting thanks to the aforementioned appalling sighting!

This example is now history of course, replaced by a supposedly state-of-the-art ATC system that works almost as well!

 

[Mawkie]

So, altogether a total of SIX different timing sections, applying in their various ways to four different signals, to protect against two different situations

Sounds complicated!

Could you offer any insight into the differences between, and uses for:

A. Draw up signals

B. Approach controlled signals

C. Speed controlled signals

 

[bramling]

Sounds complicated!

Could you offer any insight into the differences between, and uses for:

A. Draw up signals

B. Approach controlled signals

C. Speed controlled signals

A and C are essentially the same. Just that draw-up is a term used to describe certain situations where a dedicated signal is provided - often in station platforms.

An approach controlled signal is merely one which clears when a train passes a particular point, with no checking of speed. The home signal at High Barnet (NU.3) was one such example, it would clear at the point an approaching train passed the previous signal (NU.300), subject to the route being set of course.

Note that on LU the various programme machine or computer systems give the false illusion of approach control, in that signals will be called to clear when an approaching train reaches a set point. This is *not* the same thing. In such situations drivers will tend to see signals clear at the same place as that’s when the system tries to set the route, however if the signaller is working the site manually then the signal may well be seen to clear much sooner.

“Speed controlled signalling” was also used to describe a particular type of signalling, mainly on the District and Piccadilly lines, where certain stations had a large number of home signals, and the first two were generally arranged to clear at a set speed. Indeed at the time drivers were even *encouraged* to anticipate them clearing. Most of this went in the 1980s, however a few installations remained - Liverpool Street on the Central Line, and Oval / Waterloo on the Northern Line. By the end this didn’t work very well as drivers were told never to anticipate signals clearing, so the benefits weren’t seen.

 

[Goldfish62]

A and C are essentially the same. Just that draw-up is a term used to describe certain situations where a dedicated signal is provided - often in station platforms.

“Speed controlled signalling” was also used to describe a particular type of signalling, mainly on the District and Piccadilly lines, where certain stations had a large number of home signals, and the first two were generally arranged to clear at a set speed.

Is the approach to Acton Town from from the Heathrow branch an example of this? I've not been on it for a good while but in the past I've noticed a fair number of signals very close together that clear sequentially as the train slows down on approach.

 

[Mawkie]

A and C are essentially the same. Just that draw-up is a term used to describe certain situations where a dedicated signal is provided - often in station platforms.

An approach controlled signal is merely one which clears when a train passes a particular point, with no checking of speed. The home signal at High Barnet (NU.3) was one such example, it would clear at the point an approaching train passed the previous signal (NU.300), subject to the route being set of course.

Note that on LU the various programme machine or computer systems give the false illusion of approach control, in that signals will be called to clear when an approaching train reaches a set point. This is *not* the same thing. In such situations drivers will tend to see signals clear at the same place as that’s when the system tries to set the route, however if the signaller is working the site manually then the signal may well be seen to clear much sooner.

“Speed controlled signalling” was also used to describe a particular type of signalling, mainly on the District and Piccadilly lines, where certain stations had a large number of home signals, and the first two were generally arranged to clear at a set speed. Indeed at the time drivers were even *encouraged* to anticipate them clearing. Most of this went in the 1980s, however a few installations remained - Liverpool Street on the Central Line, and Oval / Waterloo on the Northern Line. By the end this didn’t work very well as drivers were told never to anticipate signals clearing, so the benefits weren’t seen.

Thanks! Isn't LU signalling fascinating sometimes.

Out of interest, the train stop on the platform at Leicester Sq (E) that protects the short overlap isn't signalled, whereas the train stop on the platform at T5 does have a signal, and the draw up signal Arnos Grove is obviously signalled with a train stop. All three seem to serve the purpose of controlling speed so wondered what the merits of each system are.

Is the approach to Acton Town from from the Heathrow branch an example of this? I've not been on it for a good while but in the past I've noticed a fair number of signals very close together that clear sequentially as the train slows down on approach.

I've certainly motored from South Ealing to Acton Town at 40mph, until the 20mph just before the points on approach to Acton. What you describe sounds like the signalling working ordinarily - that is to say as a train in front moves through signalling sections, it allows the following train to move forward in sequence.

Having said that, Acton Town is an incredibly complex signalling area, so it depends on what train, is doing what move, from which platform, and which siding!

 

[Mojo]

Out of interest, the train stop on the platform at Leicester Sq (E) that protects the short overlap isn't signalled, whereas the train stop on the platform at T5 does have a signal, and the draw up signal Arnos Grove is obviously signalled with a train stop. All three seem to serve the purpose of controlling speed so wondered what the merits of each system are.

It was explained to me that the starter signals with timing sections were installed to enforce speed restrictions at station starters for stations that used to be part time, for example Covent Garden.

And on top of the sleeping policeman at Leicester Sq, you also have the locations with light up speed restriction signs which again serve a similar function!

 

[bramling]

It was explained to me that the starter signals with timing sections were installed to enforce speed restrictions at station starters for stations that used to be part time, for example Covent Garden.

Thanks! Isn't LU signalling fascinating sometimes.

Out of interest, the train stop on the platform at Leicester Sq (E) that protects the short overlap isn't signalled, whereas the train stop on the platform at T5 does have a signal, and the draw up signal Arnos Grove is obviously signalled with a train stop. All three seem to serve the purpose of controlling speed so wondered what the merits of each system are.

The Leicester Square one is interesting, as most overlaps on the tunnel section of the Piccadilly Line are short. However this one has the likelihood of a train stopping just behind it, hence the additional protection.

In terms of the merits of a blind train stop, it means drivers don’t have to worry about having a SPAD, so allowing a faster approach. The drawback is of course that trains might get tripped occasionally, which is uncomfortable for passengers. There’s also the very small risk that a train might come along with a defective tripcock, however that risk is sufficiently small to not be worth losing too much sleep over.

 

[Dstock7080]

It was explained to me that the starter signals with timing sections were installed to enforce speed restrictions at station starters for stations that used to be part time, for example Covent Garden.

Most stations closed at weekends or Sundays had timing sections to enforce the 5mph passing the starter;
West Brompton, Ravenscourt Park, Temple, Cannon Street, Bow Road etc.

 

[bluegoblin7]

And just to throw more confusion in, just as things were calming down: you also have draw up signals. No, not the in-platform signals (which from a signalling side pov have always been referred to as approach controlled signals in my experience), but ones that would allow a train to 'draw-up' to the next signal (often one that protects a junction), come to a stand and, when proved stopped, provide an early backlock release which then allows for a different route ahead. Harrow-on-the-Hill features plenty of these (the routes over the north junction are good examples of this), including some in not too obvious places. They perform similar functions to e.g. the former in-platform signals at Baker Street in terms of protecting the junction ahead, but also serve to lock the relevant points.

And on top of the sleeping policeman at Leicester Sq, you also have the locations with light up speed restriction signs which again serve a similar function!

Most famously installed at Edgware Road ahead of the S stock coming into service. This was a particularly complex, but clever, system which replaced the draw-up signals and would call a different overlap depending on stock type and other moves in the area, altering the relevant speed limit (and timing sections) to suit. Several blind trainstops (i.e. trainstops without signals) were provided to enforce the speed limits.

This IRSE paper explains it in far more detail for anyone interested: https://webinfo.uk/webdocssl/irse-k...hillips - London Underground Edgware Road.PDF

 

[ThameslinkUser]

This IRSE paper explains it in far more detail for anyone interested: https://webinfo.uk/webdocssl/irse-k...hillips - London Underground Edgware Road.PDF

Fascinating read; thank you!

 

[eldomtom2]

Thanks for all the responses, I'm primarily interested in clarification on this point:

Or it might be more complex, for example no less than 4.5 seconds through track circuit A, OR track circuit B occupied for 15 seconds. The speeds will depend on the physical length of the track circuits, but such a setup might mean the train either has to do (say) 20 mph through the first timing section, or if it exceeds this then it will have to come to a stand at the signal for it to clear.

So there are no signals that will fail to clear if the train stops before them?