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Combined approach and overlap track circuits

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Combined approach and overlap track circuits

Unread postby scarpa » Sun Oct 22, 2017 7:51 pm

There are some sites where
track circuits which are combined approach and overlap approaching signals protecting trailing junctions where the I.B.J. is 200 yards past the signal I have come across. Now the downside of this an 8 coach train can SPAD the signal and stop and the signalman would have no indication of the SPAD occurring. Does this meet signalling standards and is redesign ever considered?
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Re: Combined approach and overlap track circuits

Unread postby John Hinson » Mon Oct 23, 2017 2:56 am

As far as I know this arrangement is completely acceptable and considered safe except where there is a junction (or similar hazard) ahead of said signal. That requirement came about after an accident at Keymer, I believe.

Some drivers would say it is not a "down side" for SPDs to not be detected!

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Re: Combined approach and overlap track circuits

Unread postby StevieG » Mon Oct 23, 2017 8:52 am

scarpa wrote:There are some sites where
track circuits which are combined approach and overlap approaching signals protecting trailing junctions where the I.B.J. is 200 yards past the signal I have come across. Now the downside of this an 8 coach train can SPAD the signal and stop and the signalman would have no indication of the SPAD occurring. Does this meet signalling standards and is redesign ever considered?

A little confused here scarpa.
Where you say "... combined approach and overlap approaching signals protecting trailing junctions... ", you are referring to where a signal protects a trailing junction, one TC forms the approach and the overlap of that signal, not the signal in rear, aren't you? [ Personally I don't recall seeing such a combined TC at any signal protecting other than plain line, but don't know whether it's never been allowed at a controlled or Semi signal.]
Then, your main point of the query seems to be signalman non - awareness of a SPAD that doesn't go beyond the overlap unless the driver reports him/ herself, but that is a concern with or without a trailing junction ahead.
For a signal protecting a trailing junction, I would expect at least a TPWS TSS fitment at the signal, which, if there was a combined berth/overlap TC, would not alarm to the signaller AFAIK, but the TSS should stop the train, in which case drivers are required to report the occurrence before resetting, making it possible to proceed, for which they should also get signaller authority before moving of course.
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Re: Combined approach and overlap track circuits

Unread postby Mark Lamb » Mon Oct 23, 2017 9:33 am

I would think that this is not acceptable to current standards (and possibly not before too, depending on the design office). Consider a trailing junction - main line, and branch line. In a track circuited area, with colour light signals, it is usual to prove the overlap track circuit clear for the main line signal in the controls of the branch line signal, and vice-versa. Now if two trains are approaching the junction on both lines - and the signal is cleared for the main line, if a SPAD occurs on the branch line, the main line signal would revert to danger immediately the SPAD occurred. This is called flank protection.
Correct me if I'm wrong, but not all design offices used to apply this principle (Ladbrook Grove accident didn't have flank protection, for instance), quoting track circuit failures as causing more risk than providing this feature (signaller having to call a train past the signal at danger, without the protection of the interlocking). If the track circuits are combined, then this feature can't be provided.
On plain line, there are many examples of the track circuit from the overlap of the previous signal extending to the overlap of the signal in question. Note that this is not a measured berth track, though - just continuous track circuiting.

Regards,

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Re: Combined approach and overlap track circuits

Unread postby scarpa » Mon Oct 23, 2017 10:01 am

Correct Stevie G applies to the signal protecting the branch. Signal has TPWS but a spad would not cause the main line signal to revert to danger. The end of the Branch combined track circuit is clear of the mainline. Signals controlled obviously very busy junctions involved .Come across two sites so far .
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Re: Combined approach and overlap track circuits

Unread postby Mike Hodgson » Mon Oct 23, 2017 10:19 am

I don't really see the problem. If a train SPADs a signal but stops in the overlap, although it's a rule breach it's still a safe situation - that's the whole purpose of the overlap. Is our concern only that the signalman is unaware and the driver doesn't fess up?

Why would he stop in the overlap ? Presumably belated recognition of the danger aspect, he wouldn't have stopped if he had completely failed to see it. So he's safe and yes, he remains undetected as long as he stays put. He has route knowledge and knows there is a risk of converging collision if he restarts. But short of walking or setting back to find the aspect now showing proceed, how can he resume his journey without dropping himself in it?
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Re: Combined approach and overlap track circuits

Unread postby John Hinson » Mon Oct 23, 2017 10:50 am

Mike Hodgson wrote:I don't really see the problem. If a train SPADs a signal but stops in the overlap, although it's a rule breach it's still a safe situation - that's the whole purpose of the overlap. Is our concern only that the signalman is unaware and the driver doesn't fess up?

Why would he stop in the overlap ? Presumably belated recognition of the danger aspect, he wouldn't have stopped if he had completely failed to see it. So he's safe and yes, he remains undetected as long as he stays put. He has route knowledge and knows there is a risk of converging collision if he restarts. But short of walking or setting back to find the aspect now showing proceed, how can he resume his journey without dropping himself in it?

The importance of being aware at a junction is for the signalman to be able to take the necessary action to prevent a collision in a timely manner. As you say, there is little that can be done on plain line.

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Re: Combined approach and overlap track circuits

Unread postby Craig » Mon Oct 23, 2017 7:00 pm

Mike Hodgson wrote:I don't really see the problem. If a train SPADs a signal but stops in the overlap, although it's a rule breach it's still a safe situation - that's the whole purpose of the overlap. Is our concern only that the signalman is unaware and the driver doesn't fess up?

Why would he stop in the overlap ? Presumably belated recognition of the danger aspect, he wouldn't have stopped if he had completely failed to see it. So he's safe and yes, he remains undetected as long as he stays put. He has route knowledge and knows there is a risk of converging collision if he restarts. But short of walking or setting back to find the aspect now showing proceed, how can he resume his journey without dropping himself in it?


There is a problem in that the latest standards allow for a set of points within an overlap to be out of correspondence with the signal protecting the section leading up to the overlap showing a proceed aspect, if the train SPAD's the signal immediately before and encroaches onto the overlap it could be onto a set of points with no detection, however this digresses from the original topic as the overlap is a separate track circuit!
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Re: Combined approach and overlap track circuits

Unread postby Ashley Hill » Mon Oct 23, 2017 9:09 pm

Mike Hodgson wrote:I don't really see the problem. If a train SPADs a signal but stops in the overlap, although it's a rule breach it's still a safe situation - that's the whole purpose of the overlap. Is our concern only that the signalman is unaware and the driver doesn't fess up?


Whether the driver who spads puts his nose past the signal or stops completely within the overlap it's pretty hard to hide it these days and there are severe penalties for those who try to. TPWS intervention, AWS or late breaking/misjudgment all show up on the OTMR. These are also randomly downloaded and examined for each driver a set number of times per year and anomalies investigated. Add to that the front end cctv on many trains (also recorded), travelling managers with mobiles or even passengers tweeting (yes it's happened) a driver is better to '"fess up!"
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Re: Combined approach and overlap track circuits

Unread postby Mark Lamb » Tue Oct 24, 2017 9:38 am

Further to the original question, at Mostyn SB on the North Wales coast (recently abolished), the Up berth track circuit was combined with the 'overlap' track circuit up to the Home 2 signal. I say 'overlap' because it wasn't a true overlap - it wasn't a measured track circuit and therefore wasn't used as the block acceptance point.

I haven't a clue why it was like this, and it didn't really help the signaller because he couldn't return his Home 1 signal (mechanical) to danger until he was sure the train had passed it!

Regards,

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Re: Combined approach and overlap track circuits

Unread postby Fast Line Floyd » Tue Oct 24, 2017 12:04 pm

Craig wrote:
There is a problem in that the latest standards allow for a set of points within an overlap to be out of correspondence with the signal protecting the section leading up to the overlap showing a proceed aspect, if the train SPAD's the signal immediately before and encroaches onto the overlap it could be onto a set of points with no detection, however this digresses from the original topic as the overlap is a separate track circuit!

This applies to trailing points only and is not a problem except with HPSA machines as the worst that could happen at the points is that they are run through. In the case of HPSA machines it is not permitted.
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Re: Combined approach and overlap track circuits

Unread postby S&TEngineer » Wed Oct 25, 2017 3:30 pm

I think you will find this was quite common practice on the former BR-SR lines. Examples can be found here: http://www.davidheyscollection.com/user ... ot-Box.jpg and here: http://www.wbsframe.mste.co.uk/public/p ... iagram.jpg
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Re: Combined approach and overlap track circuits

Unread postby scarpa » Wed Oct 25, 2017 7:09 pm

You are quite correct S&T engineer both sites are on Southern lines. Looking at the Streatham signalbox diagram the depiction of the signals is somewhat different you would have found on diagrams in later years. Was this the standard for Southern Region diagrams anyone know of any history?
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Re: Combined approach and overlap track circuits

Unread postby Fast Line Floyd » Wed Oct 25, 2017 9:51 pm

scarpa wrote:You are quite correct S&T engineer both sites are on Southern lines. Looking at the Streatham signalbox diagram the depiction of the signals is somewhat different you would have found on diagrams in later years. Was this the standard for Southern Region diagrams anyone know of any history?

It is a Westinghouse diagram supplied as part of the contract for the power frame, all of the boxes in that contract and indeed all of the contracts for the era with WBS were of the same style.
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Re: Combined approach and overlap track circuits

Unread postby S&TEngineer » Thu Oct 26, 2017 9:04 am

The combined berth and overlap arrangement was perpetuated until the power schemes of the mid 1980s. Once the BR regions lost their autonomy with the start of 'Sectorisation' the BR Board started to impose strict BR wide standards and former regional practices were overidden.
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