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Double-Blocking

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O L Leigh

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I was wondering whether any of the wise heads on this forum could explain to me why it is that, in the era of TPWS, there are still stations that appear to use double-blocking. Two that come most readily to mind are Cambridge (where you cannot access P4 on the Down if there is a movement signalled in or out of P5/6) and Newport Wales (where you cannot access either P3 or P4 if there is another train on the opposing platform with the route set out of the platform).

Now I understand that you want to protect the point of conflict in the event that a driver makes a total horlicks of stopping and SPADs the signal, but as signals at these locations are equipped with TPWS anyway the chances of a SPAD resulting in a collision is low. This also contrasts with junctions such as Proof House where you similarly have points of conflict where a train passing a signal could potentially cause a collision but where double-blocking is not used.

It's quite irritating to be sat at a red signal looking at your vacant platform but knowing that you can't access it until after another train has finished using some other piece of track somewhere near to the bit that you're about to occupy. Obviously it causes capacity constraints and means that it is not possible to fully utilise all the available platform space which can introduce/exacerbate delays. To this humble driver's eyes, it's a daft idea that causes more problems than it solves. So what is it that I'm missing? What salient factor can explain why double-blocking is persisted with?
 
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Ianno87

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TPWS does not prevent a SPAD (although the likelihood is reduced) - it merely makes sure that a train that does SPAD stops within the signal overlap.

So in Cambridge's example, it is the signal overlaps that prevent the moves you would describe, so TPWS would not prevent a theoretical collision if the simultaneous movements were permitted.
 

O L Leigh

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Yes I am aware that TPWS does not prevent SPADs, but it does stop a train automatically if it SPADs a fitted signal, which all of the signals in question are. The reason for my query is to do with it’s inconsistent application. If it’s OK not to double-block at locations where you can get as close to the point of conflict (if not closer) as at Cambridge (for example), why are we still doing it at locations where it creates issues with capacity?
 

Surreytraveller

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I am not familiar with Cambridge, but could the point of conflict be within the overlap?
Similar to platform 4 at East Croydon - if a train is signalled north out of platform 5, a train cannot enter platform 4, as the points are within the overlap
 

DanNCL

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Double blocking is used for all signals between Pelaw Metro Junction and Sunderland. Reason it’s used there is because the Metro units sharing the line with national rail aren’t built to mainline crashworthiness standards, so it’s effectively an additional measure to make a collision even less likely than the already incredibly small chance it had of happening. Double blocking is expected to be removed from the line once the Metro fleet has been replaced in the next few years, as the new Stadler fleet will be built to mainline crashworthiness standards, and removal of double blocking is required to allow the planned 6TPH Metro service from 2024.
 

GB

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The conflict point between P4 and P5/6 at Cambridge is well within the protecting signal overlap which makes the risk of collision a lot higher than "low"...particularly for through trains.
 

TheEdge

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I'm currently route learning Cambridge but my trainer is an ex-NR manager and we were talking about double blocking in general and approaching Ely North Junction in particular despite developments in signaling. Apparently it all stems from a loss of faith in conflict points and stopping trains at the last signal before the conflict point after the Newton crash.
 

baz962

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Couple of places on the MML. Can't remember all of them , but in one place it's because the red is very close to the previous yellow and at linespeed you wouldn't stop for the red .
 

Tomnick

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This isn't, to my mind, "double blocking" - it's just a conflicting route within the overlap of the 'exit' signal, which is a concept dating back to the first use of clearing points over a century ago! The length of the overlap, plus provision and length of any reduced/restricted overlap, is governed by the applicable design standards.

Double red protection - still not double blocking! - is a more recent concept if I'm not mistaken, generally intended to deal with the risk where TPWS associated with the protecting signal might not be able to stop a train from linespeed before the conflict point. Conditional double red protection, for example, is provided on the approach to Slade Lane Junction, where the second signal in rear of the junction won't clear if there's no route set from the next signal, until the approaching train has passed over the OSS associated with that signal, thus proved to be under control.
 

Ianno87

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I am not familiar with Cambridge, but could the point of conflict be within the overlap?
Similar to platform 4 at East Croydon - if a train is signalled north out of platform 5, a train cannot enter platform 4, as the points are within the overlap

The conflict point between P4 and P5/6 at Cambridge is well within the protecting signal overlap which makes the risk of collision a lot higher than "low"...particularly for through trains.

Yes, that's the point at Cambridge - the turnout into P5/6 falls within the Overlap of the P4 end signal.

For P1 and P4 "head on" simultaneous arrivals, if both arriving trains enter their respective overlaps there is a potential collision. (In both cases a restricted (or swung) overlap is available so one platform can be entered with a train standing in the other.
 

dk1

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I'm currently route learning Cambridge but my trainer is an ex-NR manager and we were talking about double blocking in general and approaching Ely North Junction in particular despite developments in signaling. Apparently it all stems from a loss of faith in conflict points and stopping trains at the last signal before the conflict point after the Newton crash.
Ely North Jcn was double blocked from resignalling in the early 90s following several collisions at single lead junctions & has therefore never fully shown its potential. It’s only fairly recently that the West Curve has finally been allowed to operate bi-di.
 

Bald Rick

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To flesh out the Cambridge example some more, it’s not double blocking. For a train to be signalled to CA175, the overlap must be free. The points leading into Platforms 5 and 6 (1073 points) are within that overlap. Therefore if a route is set into or out of platforms 5 and 6, then a route can not be set up to CA175.

Similarly, if a route is set up to CA175, then a route can not be set into or out of platforms 5 or 6, although I suspect there is a timed release for this to deal with a terminating train.

There is only about 25 metres from CA175 to the conflict point at 1073 points, which is not enough for TPWS to stop a train running at the linespeed of 35mph. In fact it would be at the conflict point not long after the brakes were fully applied.
 
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Surreytraveller

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To flesh out the Cambridge example some more, it’s not double blocking. For a train to be signalled to CA175, the overlap must be free. The points leading into Platforms 5 and 6 (1073 points) are within that overlap. Therefore if a route is set into or out of platforms 5 and 6, then a route can not be set up to CA175.

Similarly, if a route is set up to CA175, then a route can not be set into or out of platforms 5 or 6, although 8 suspect there is a timed release for this to deal with a terminating train.

There is only about 25 metres from CA175 to the conflict point at 1073 points, which is not enough for TPWS to stop a train running at the linespeed of 35mph.
I was going to agree, its not double blocking. I believe (correct me if I'm wrong), double blocking is where two blocks, including the overlap (instead of the normal one block), is clear.
It might be used, for example, when a particular track circuit cannot be relied upon, such as if there is contamination on the railhead preventing a track circuit from operating as it should.
 

craigybagel

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I was wondering whether any of the wise heads on this forum could explain to me why it is that, in the era of TPWS, there are still stations that appear to use double-blocking. Two that come most readily to mind are Cambridge (where you cannot access P4 on the Down if there is a movement signalled in or out of P5/6) and Newport Wales (where you cannot access either P3 or P4 if there is another train on the opposing platform with the route set out of the platform).
The Newport example is particularly frustrating as for the last couple of months the signal protecting those platforms has been held at danger for all trains approaching until they've passed over the OSS, regardless of whether or not the platforms are occupied or if the route beyond is clear. It's like an approach controlled signal - except there's no obvious need for it and before the recent changes it used to routinely be green.

The signal in question is at the end of Newport tunnel, making it extra difficult to judge it's location meaning it has to be approached even more cautiously than would be the norm for a signal at danger.
 

O L Leigh

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OK, so we're talking about overlaps. So to take the second part of my complaint, why is it permitted to get so close to a point of conflict at other locations such as Proof House Jn?

The Newport example is particularly frustrating as for the last couple of months the signal protecting those platforms has been held at danger for all trains approaching until they've passed over the OSS, regardless of whether or not the platforms are occupied or if the route beyond is clear. It's like an approach controlled signal - except there's no obvious need for it and before the recent changes it used to routinely be green.

Yes, I have noticed that too. However, that is a slightly different issue. The real problems with P3/4 at Newport is that you can't access one platform while the other is occupied by a train going in the same direction.

To illustrate, a few days back there was a queue of four trains all trying to call at Newport; a delayed TfW Shrewsbury (I think), an XC Nottingham, a TfW Manchester and then the GWR London. Even though the platform allocation for each alternated between P3 and P4, each train was unable to access it's platform until the previous one had departed and cleared the outbound overlap. From an operational standpoint it effectively reduced that side of the station to a single platform and only ensured that delays were imported onto each of the following services.
 

craigybagel

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OK, so we're talking about overlaps. So to take the second part of my complaint, why is it permitted to get so close to a point of conflict at other locations such as Proof House Jn?



Yes, I have noticed that too. However, that is a slightly different issue. The real problems with P3/4 at Newport is that you can't access one platform while the other is occupied by a train going in the same direction.

To illustrate, a few days back there was a queue of four trains all trying to call at Newport; a delayed TfW Shrewsbury (I think), an XC Nottingham, a TfW Manchester and then the GWR London. Even though the platform allocation for each alternated between P3 and P4, each train was unable to access it's platform until the previous one had departed and cleared the outbound overlap. From an operational standpoint it effectively reduced that side of the station to a single platform and only ensured that delays were imported onto each of the following services.
Very true. I only mention it because it means all trains are approaching 3 and 4 so slowly now it seems to make the restrictive overlaps even more unnecessary than they already are

And also because I was hoping someone would be able to explain why these changes were deemed necessary when we used to be able to approach Newport on greens. It's making the problems you've described even worse as it's taking a minute longer for each train to arrive at the platform.
 

Bald Rick

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Which signals at Proof House? The only one I can think of is on the Up Stour just before the Stours to Derbys double Junction. That overlap is longer, although I have a feeling it might swing onto the Up Derby.
 

O L Leigh

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And also because I was hoping someone would be able to explain why these changes were deemed necessary when we used to be able to approach Newport on greens. It's making the problems you've described even worse as it's taking a minute longer for each train to arrive at the platform.

That's precisely the reason for starting this thread. It seems that, at some locations at least, signalling is now constricting capacity more than platform occupancy, which has an impact on the service.

Which signals at Proof House? The only one I can think of is on the Up Stour just before the Stours to Derbys double Junction. That overlap is longer, although I have a feeling it might swing onto the Up Derby.

I'm thinking specifically the one on the Up Derby that protects the junction with the Vauxhall Lines coming off the viaduct (sorry, signal number not immediately at hand).

I'm currently route learning Cambridge but my trainer is an ex-NR manager and we were talking about double blocking in general and approaching Ely North Junction in particular despite developments in signaling. Apparently it all stems from a loss of faith in conflict points and stopping trains at the last signal before the conflict point after the Newton crash.

Good lord!! Newton was, what, early 1990's...? Certainly pre-TPWS. I can't quickly find any information about how far the overrun was and therefore get any feeling for whether TPWS would have prevented the collision.

I have often wondered how the overlaps are calculated and what allowance they make for improvements in train braking capabilities and the fact that TPWS will apply the brake immediately.
 

craigybagel

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Newton was a SASSPAD, so surely the signalling issues mentioned in this thread wouldn't have made a difference in that accident, as the train that had the SPAD would have been deemed to have safely stopped at the signal and it therefore safe for the other train to be signalled across the junction.

(In fact, before the SPAD the two trains actually were heading into Newton from the same direction).
 

Bald Rick

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I'm thinking specifically the one on the Up Derby that protects the junction with the Vauxhall Lines coming off the viaduct (sorry, signal number not immediately at hand).

NS139. That has a reduced overlap of around 80 metres overlap, which is sufficient for the 30mph linespeed. A train passing the TSS(1) grids at the signal at linespeed* would be stopped before the conflict point with the Up Vauxhall.

*assuming there are OSS(2) grids on approach, it would be a very unusual set of circumstances for a driver to be under the OSS speed but at linespeed by the TSS grids at signal.

(1) Train Stop Sensor
(2) Over Speed Sensor
 

craigybagel

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Maybe I have misunderstood what you mean, but the trains were heading in opposite directions and the collision was head-on.
I phrased it badly. The 303 that had the SPAD that caused the accident arrived from the Glasgow direction into Newton and terminated. It then departed again in the opposite direction on its return journey - and it was as it was departing that it had the SPAD, hitting the 314 that was also coming in from Glasgow.

My point was that overlaps shouldn't have had any bearing on the signalling prior to the SPAD, as when the 303 arrived at Newton on the down platform the signaller would have been clear to then signal the 314 into Newton on the up.
 

Wilts Wanderer

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Is the P3/4 restriction at Newport a recent change? I remember with the old layout P3 had a mid platform signal which you could approach while the previous train was still exiting the platform at the far end.

Either way it probably doesn’t bode well for performance when TfW commence their hourly service from Crosskeys in December, which will use P4 and the UML to/from Gaer Jcn in both directions with any conflicting Up trains from Cardiff using the DML through Newport tunnel into P3, and any further conflicting down train from P2 pushed across to the RL towards Ebbw Jcn.
 

Falcon1200

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I phrased it badly. The 303 that had the SPAD that caused the accident arrived from the Glasgow direction into Newton and terminated. It then departed again in the opposite direction on its return journey - and it was as it was departing that it had the SPAD, hitting the 314 that was also coming in from Glasgow.

My point was that overlaps shouldn't have had any bearing on the signalling prior to the SPAD, as when the 303 arrived at Newton on the down platform the signaller would have been clear to then signal the 314 into Newton on the up.

Thanks, now I understand !

I recall my colleagues at work, seeing the proposed track layout at Newton before the work was carried out (which was mainly to benefit Inter-City by raising line speeds on the through lines) saying it was a recipe for disaster; Tragically of course they were proved right, and the layout was subsequently rebuilt again to eliminate that single lead junction.
 

D6130

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When I first saw the title of this thread, I thought it was about double-blocking in main line running. At the time of my retirement nine years ago, double-blocking was in force on the WCML North of Preston for EPS (Enhanced Permissible Speed - i.e. tilting Pendolino and Voyager) services. Coupled with the presence of the two level crossings at Hest Bank and Bolton-le-Sands, this invariable caused frustrating delays to our Lancaster/Morecambe to Skipton/Leeds services....although, when there were no TSRs, the time lost could usually be recovered between Gargrave and Skipton.
 

louis97

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Is the P3/4 restriction at Newport a recent change? I remember with the old layout P3 had a mid platform signal which you could approach while the previous train was still exiting the platform at the far end.

Either way it probably doesn’t bode well for performance when TfW commence their hourly service from Crosskeys in December, which will use P4 and the UML to/from Gaer Jcn in both directions with any conflicting Up trains from Cardiff using the DML through Newport tunnel into P3, and any further conflicting down train from P2 pushed across to the RL towards Ebbw Jcn.
Definitely recent, the signals at the East end of Platform 3 and 4 are set back down the platform with overlaps of 74 and 78 metres respectively which stand clear of the crossovers at the East end. So any restriction on an arrival on P3/4 whilst a departure from the other isn't a restriction because of the overlaps themselves. I wonder if it is connected to the approach control on NT1070 on the Up Main on the approach to Newport from Cardiff.
 

craigybagel

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Definitely recent, the signals at the East end of Platform 3 and 4 are set back down the platform with overlaps of 74 and 78 metres respectively which stand clear of the crossovers at the East end. So any restriction on an arrival on P3/4 whilst a departure from the other isn't a restriction because of the overlaps themselves. I wonder if it is connected to the approach control on NT1070 on the Up Main on the approach to Newport from Cardiff.
The approach control into Newport has no obvious reasons for existing. It applies even for trains going straight on into P4 with no change of linespeed - and even if going over the crossover into P3 it's only a 5mph reduction.

It also only started appearing without warning a few months ago - until then getting greens all the way through Newport was possible.
 

Halish Railway

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There’s another example at Shipley - 1A19 0843 Bradford Forster Square to London Kings Cross is held outside of Shipley South Jn until 2D37 0822 Ilkley to Bradford Forster Square has cleared Shipley East Jn as the booked 10 car Azuma needs to foul the signal towards the end of the platform in order to fit the whole train into the recently extended platform.
 

Juniper Driver

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St Deny's down Road is good for that. I don't really have a problem with it, and the signals are close together.
Rayne's Park is technically approach controlled, but you obviously can't take that for granted... Probably many other places, I can't think of at the moment.

Woking, though, probably for many reasons.
 
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