TPWS Every Signal

[bengley]

Yep I’d thought that, until reading that the AWS was also isolated. Of course that would be absolutely unthinkable these days, but was apparently permissible practice at the time!

Not completely unthinkable

 

[LowLevel]

Without context, possibly. In practice a lot of activations the driver is already braking for the signal, so there’s no saying whether they’d even have passed the signal, never mind by enough to cause a major incident.

Take for example on GW, where the IETs are fitted with TPWS and ATP.

ATP is continually computing the “safe” speed for the train to be doing approaching the signal, with distance to go etc; and it’ll warn the driver to brake harder if they’re cutting it fine, and it will step in and apply the brakes itself if the driver doesn’t improve their retardation.

But despite that, we get IETs tripped up on TPWS approaching signals. ATP hasn’t warbled because on the continually computed curve the train is fine, but TPWS wants the driver to be doing less - and the set speeds are, officially at least, blind. They’re not divulged to driving grades, at least at my TOC as “it’d be like offering a sugar lump to a racehorse”.

Interestingly I know of one lineside board, shaped like an advanced warning indicator for a speed restriction, but with TPWS and the speed it is set at below it. If I remember rightly it's on the down approaching Thetford. I assume there have been a number of activations there.

 

[High Dyke]

Isn't every brake intervention caused by the TPWS intervention a possible major incident averted ?

Probably, to some extent. In the case of the Over speed sensors (OSS) that may be different. I was involved in the activation of an OSS a number of years ago. The driver was travelling too fast, but the signal the unit was applicable to had cleared on approach control - probably something the driver wasn't expecting. The signalling was actually working in the correct order, and it left me bemused to see the signal clear to a proceed aspect, but the train stop and the driver contact me to advise what had happened.

 

[Domh245]

A perhaps more interesting follow-up question is how many near misses would have become "just" normal incidents with ATP - cases where if not for some stroke of fortune there could have been fatalities but ATP would have turned it into just a tea, no biscuit meeting. Wooton Bassett comes to mind as an example big incident were luck prevented a much worse outcome, although that was another AWS/TPWS isolated incident so an ATP system would have been ineffective there

 

[43066]

although that was another AWS/TPWS isolated incident so an ATP system would have been ineffective there

AIUI ATP and AWS/TPWS are completely independent, so isolating AWS/TPWS wouldn’t, of itself, render ATP inactive (I have no idea whether GWR have both systems operating in tandem on the ATP sections of track, or switch between the two according to location).

At Southall (which was pre TPWS) both AWS and ATP systems were isolated, so ATP provided no protection, and we don’t know whether the action of cancelling the AWS would have alerted to the driver to restrictive aspects (a major flaw with AWS is that the action of cancelling quickly becomes thoughtless when you’re running on restrictive aspects for extended periods).

 

[Efini92]

On the contrary, on the routes I sign, there are a couple of high risk controlled signals where TPWS is not fitted where you'd think it should be.

Cove LC on the WCML is one of them. No overspeed or train stop loops on a 125mph line, approaching a CCTV level crossing. Quite extraordinary if you ask me!

Does it have flank protection?

 

[Domh245]

AIUI ATP and AWS/TPWS are completely independent, so isolating AWS/TPWS wouldn’t, of itself, render ATP inactive (I have no idea whether GWR have both systems operating in tandem on the ATP sections of track, or switch between the two according to location).

At Southall (which was pre TPWS) both AWS and ATP systems were isolated, so ATP provided no protection, and we don’t know whether the action of cancelling the AWS would have alerted to the driver to restrictive aspects (a major flaw with AWS is that the action of cancelling quickly becomes thoughtless when you’re running on restrictive aspects for extended periods).

In the Wootton Basset case, the TPWS became inoperative because operating the AWS isolating cock had also prevented the TPWS from working (whether this only applies to kettles or also to normal stock I don't know!) but it's another case of "if the installed system was working properly, the incident would have been lessened" - only one incident was fatal and the other wasn't. An ATP system onboard a steam engine would probably have also been subjected to inadvertently being bypassed (at least, based on the behaviours of that crew and slightly less stringent regulations when the loco was certified) but it did get me thinking about other near misses.

 

[43066]

Interestingly I know of one lineside board, shaped like an advanced warning indicator for a speed restriction, but with TPWS and the speed it is set at below it. If I remember rightly it's on the down approaching Thetford. I assume there have been a number of activations there.

Unless I’m misunderstanding what you’re describing, that’s standard for overspeed grids protecting large PSR reductions. You get the “Morpeth board” (yellow bordered warning triangle) and then the grids will be located some way before the PSR commencement board.

Every so often signage (and grids) might be moved and this usually generates a spate of activations/interventions. I sign a location just like this where the speed drops from 120 to 80. When it was resignalled the grids were moved but the signage wasn’t, which tripped a load of drivers up when they didn’t adapt their braking points (I believe they were exonerated).

In the Wootton Basset case, the TPWS became inoperative because operating the AWS isolating cock had also prevented the TPWS from working (whether this only applies to kettles or also to normal stock I don't know!) but it's another case of "if the installed system was working properly, the incident would have been lessened" - only one incident was fatal and the other wasn't. An ATP system onboard a steam engine would probably have also been subjected to inadvertently being bypassed (at least, based on the behaviours of that crew and slightly less stringent regulations when the loco was certified) but it did get me thinking about other near misses.

Ah ok I’m with you. Presumably if both systems had been fitted and operable the (almost lethal) conflict would have been prevented by either, but with less risk of a spad happening in the first place with ATP, because it provides continuous speed supervision, rather than “speed traps”.

“TPWS+” (additional grids on 90mph + lines set at higher speeds and further back from conflict points) muddy the waters even further in favour of TPWS.

In terms of AWS/TPWS the system are alloyed so knocking out AWS will always knock out TPWS. Some stock has an additional isolation switch for TPWS (separate to the usual temporary/permanent overrides) so that TPWS can be completely knocked out but AWS left active.

== Doublepost prevention - post automatically merged: 12 Oct 2020 ==

Not completely unthinkable

 

[LowLevel]

Unless I’m misunderstanding what you’re describing, that’s standard for overspeed grids protecting large PSR reductions. You get the “Morpeth board” (yellow bordered warning triangle) and then the grids will be located some way before the PSR commencement board.

Every so often signage (and grids) might be moved and this usually generates a spate of activations/interventions. I sign a location just like this where the speed drops from 120 to 80. When it was resignalled the grids were moved but the signage wasn’t, which tripped a load of drivers up when they didn’t adapt their braking points (I believe they were exonerated).




Ah ok I’m with you. Presumably if both systems had been fitted and operable the (almost lethal) conflict would have been prevented by either, but with less risk of a spad happening in the first place with ATP, because it provides continuous speed supervision, rather than “speed traps”.

“TPWS+” (additional grids on 90mph + lines set at higher speeds and further back from conflict points) muddy the waters even further in favour of TPWS.

In terms of AWS/TPWS the system are alloyed so knocking out AWS will always knock out TPWS. Some stock has an additional isolation switch for TPWS (separate to the usual temporary/permanent overrides) so that TPWS can be completely knocked out but AWS left active.

== Doublepost prevention - post automatically merged: 12 Oct 2020 ==

I don't think I've ever seen a warning board specifically with the TPWS trip speed on it though apart from this one? I don't think there are any others on the routes I sign anyway. (Just to be clear - the board has the letters "TPWS" and the speed in MPH that it trips at written on it, rather than the speed restriction that will apply).

 

[43066]

I don't think I've ever seen a warning board specifically with the TPWS trip speed on it though apart from this one? I don't think there are any others on the routes I sign anyway. (Just to be clear - the board has the letters "TPWS" and the speed in MPH that it trips at written on it, rather than the speed restriction that will apply).

Thanks for clarifying.

Agreed. I’ve never come across that either, although it doesn’t seem a bad idea. The speeds the grids protecting PSRs trigger at vary (for obvious reasons) and are usually a poorly guarded secret!

In my experience the grids protecting signals are usually set at around 45mph for standard TPWS (grids close to the red) or 65ish for TPWS+ (grids before the single yellow).

 

[edwin_m]

In the Wootton Basset case, the TPWS became inoperative because operating the AWS isolating cock had also prevented the TPWS from working (whether this only applies to kettles or also to normal stock I don't know!).

Can't speak for this specific case but I know TPWS was designed to replace the AWS equipment (with the addition of a control panel) and provide both functions. In fact the W in TPWS stands for Warning and refers to the AWS function. So I would expect that both functions would use the same brake valve, and therefore the same isolating cock would render both of them ineffective.

 

[notadriver]

ETCS soon anyway.

 

[Deepgreen]

Fundamentally it's cost/benefit analysis based on many factors, most importantly the number of passengers involved and likely outcome of a SPAD. Thus, very crudely, a lightly-used location with low speeds will be less likely to qualify than a high-speed, busy place.

 

[Tomnick]

In the Wootton Basset case, the TPWS became inoperative because operating the AWS isolating cock had also prevented the TPWS from working (whether this only applies to kettles or also to normal stock I don't know!) but it's another case of "if the installed system was working properly, the incident would have been lessened" - only one incident was fatal and the other wasn't. An ATP system onboard a steam engine would probably have also been subjected to inadvertently being bypassed (at least, based on the behaviours of that crew and slightly less stringent regulations when the loco was certified) but it did get me thinking about other near misses.

If I'm not mistaken, neither the AWS nor the TPWS was isolated prior to the Wootton Bassett incident - both systems remained operational. The isolating cock that the fireman operated was part of the link between those systems and the braking system, so any brake demand was rendered ineffective. It was, again if I'm not mistaken, a deliberate act to prevent the train coming to a stand after the driver was slow to cancel the AWS, but was inadvertently left isolated. It's certainly not something that you'd expect to happen on 'normal' stock!

In my experience the grids protecting signals are usually set at around 45mph for standard TPWS (grids close to the red) or 65ish for TPWS+ (grids before the single yellow).

Are there any/many examples of overspeed sensors positioned before the signal in rear of the one that they're associated with? TPWS+ is obviously further out from the signal than the standard OSS, but I don't know if I've ever come across one positioned before the single yellow. There's one where, as I understand it, there needs to be an OSS before the single yellow to stop the fastest train before the conflict point, and in that case the signal in rear is held at danger until you've passed over the extra OSS (unless there's a route set through the junction obviously).

 

[43066]

It was, again if I'm not mistaken, a deliberate act to prevent the train coming to a stand after the driver was slow to cancel the AWS, but was inadvertently left isolated. It's certainly not something that you'd expect to happen on 'normal' stock!

Exactly the same in an HST, you have to walk back to a dark room behind the cab and pull a cock (no sniggering at the back), in order to isolate the AWS/TPWS. The system might well still be “active” but it won’t stop the train, so the effect is the same.

Re. Wooten Bassett it was done entirely deliberately, as I recall, in order to prevent the train coming up in a heap as an inconvenient moment. Hence the driver getting a suspended prison sentence.

Are there any/many examples of overspeed sensors positioned before the signal in rear of the one that they're associated with? TPWS+ is obviously further out from the signal than the standard OSS, but I don't know if I've ever come across one positioned before the single yellow. There's one where, as I understand it, there needs to be an OSS before the single yellow to stop the fastest train before the conflict point, and in that case the signal in rear is held at danger until you've passed over the extra OSS (unless there's a route set through the junction obviously).

I know of one location where the speed drops from 90 to 50 (IIRC), the TPWS+ grids straddle the magnet for the single yellow and are set at 62.5mph. I know that, because I know two people who tripped it, one of whom posts on here haha (and no it wasn’t me) .

 

[rower40]

The other snag about TPWS everywhere is the ripple effect if a signal is placed back to danger. Consider 5 TPWS-fitted signals, numbered 1-5 in the direction of travel. Train is approaching signal 1, all are at green. Signaller realises that he's cleared 5 in error, and replaces it. Without TPWS, 5 goes to red, 4 to yellow, 3 to double yellow, 2 and 1 stay at green. With TPWS; 5 goes to red, but it takes a second or two for the TPWS loops to energise. To protect the (momentary) failure of red aspect at signal 5, signal 4 has to go to red. The same happens there - 3 has to go to red too. All the way back until the driver sees a flick of aspect back at signal 1.

 

[Tomnick]

Exactly the same in an HST, you have to walk back to a dark room behind the cab and pull a cock (no sniggering at the back), in order to isolate the AWS/TPWS. The system might well still be “active” but it won’t stop the train, so the effect is the same.

Re. Wooten Bassett it was done entirely deliberately, as I recall, in order to prevent the train coming up in a heap as an inconvenient moment. Hence the driver getting a suspended prison sentence.

That's the difference - at Wootton Bassett, it was done deliberately (by the fireman?) to keep the train moving, as was apparently routinely done. You might be able to isolate it in a similar manner on an HST, but you're certainly unlikely to go and do it on the move when you miss the AWS!

I know of one location where the speed drops from 90 to 50 (IIRC), the TPWS+ grids straddle the magnet for the single yellow and are set at 62.5mph. I know that, because I know two people who tripped it, one of whom posts on here haha (and no it wasn’t me) .

I know one location, or at least I think/hope I do, where the speed drops from 80 to 50, where the TPWS+ loops (for the next signal) are immediately beyond the single yellow and straddle the magnet for the 50mph warning board. Nice castle as a backdrop too.

The other snag about TPWS everywhere is the ripple effect if a signal is placed back to danger. Consider 5 TPWS-fitted signals, numbered 1-5 in the direction of travel. Train is approaching signal 1, all are at green. Signaller realises that he's cleared 5 in error, and replaces it. Without TPWS, 5 goes to red, 4 to yellow, 3 to double yellow, 2 and 1 stay at green. With TPWS; 5 goes to red, but it takes a second or two for the TPWS loops to energise. To protect the (momentary) failure of red aspect at signal 5, signal 4 has to go to red. The same happens there - 3 has to go to red too. All the way back until the driver sees a flick of aspect back at signal 1.

Is that standard? I've had a couple of TPWS-fitted signals put back to danger a little way ahead of me in recent months. I saw the second signal in rear, in each case, go back from green to double yellow as expected. The signal that was put back is, again in each case, provided with TPWS, so didn't behave as you describe. Could it be treated in a similar manner to facing points being swung in an overlap (once detection is lost, seven seconds for it to be regained in the other position before the signal in rear reverts to danger?).

 

[JN114]

The other snag about TPWS everywhere is the ripple effect if a signal is placed back to danger. Consider 5 TPWS-fitted signals, numbered 1-5 in the direction of travel. Train is approaching signal 1, all are at green. Signaller realises that he's cleared 5 in error, and replaces it. Without TPWS, 5 goes to red, 4 to yellow, 3 to double yellow, 2 and 1 stay at green. With TPWS; 5 goes to red, but it takes a second or two for the TPWS loops to energise. To protect the (momentary) failure of red aspect at signal 5, signal 4 has to go to red. The same happens there - 3 has to go to red too. All the way back until the driver sees a flick of aspect back at signal 1.

Can’t say I’ve ever heard of this before? Where is it implemented like that?

 

[bengley]

Does it have flank protection?

They don't normally fit level crossings with flank protection, no!

== Doublepost prevention - post automatically merged: 13 Oct 2020 ==

ETCS soon anyway.

I wouldn't be so sure that'll be seen as cost effective now that we don't need as much capacity.

 

[edwin_m]

Is that standard? I've had a couple of TPWS-fitted signals put back to danger a little way ahead of me in recent months. I saw the second signal in rear, in each case, go back from green to double yellow as expected. The signal that was put back is, again in each case, provided with TPWS, so didn't behave as you describe. Could it be treated in a similar manner to facing points being swung in an overlap (once detection is lost, seven seconds for it to be regained in the other position before the signal in rear reverts to danger?).

Possibly something to do with the difference between places where TPWS was retrofitted and the detection cut into the lamp proving circuit, and more recent installations where it got a separate indication back to the signaling centre?

 

[Efini92]

They don't normally fit level crossings with flank protection, no!

== Doublepost prevention - post automatically merged: 13 Oct 2020 ==


I wouldn't be so sure that'll be seen as cost effective now that we don't need as much capacity.

Apologies i didn’t realise you were on about a level crossing. I thought you meant a junction.
I don’t think it’s normal practice to equip a level crossing with TPWS. It certainly isn’t in the north west.

 

[Tomnick]

Possibly something to do with the difference between places where TPWS was retrofitted and the detection cut into the lamp proving circuit, and more recent installations where it got a separate indication back to the signaling centre?

I did wonder that, but one of my examples was under a pre-TPWS relay interlocking.