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Combined approach and overlap track circuits

Current and future British signalling (UK except Northern Ireland)

Re: Combined approach and overlap track circuits

Unread postby John Hinson » Thu Oct 26, 2017 11:53 am

S&TEngineer wrote: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.

I thought it was outlawed at junctions after the Keymer Junction incident - which would be some years prior to that. I haven't managed to find the date yet.

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Re: Combined approach and overlap track circuits

Unread postby Fast Line Floyd » Thu Oct 26, 2017 2:27 pm

John Hinson wrote:
S&TEngineer wrote: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.

I thought it was outlawed at junctions after the Keymer Junction incident - which would be some years prior to that. I haven't managed to find the date yet.

John

Indeed but that wouldn't necessarily mean a wholesale changing of track circuit arrangements on existing signalling.
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Re: Combined approach and overlap track circuits

Unread postby JG Morgan » Thu Oct 26, 2017 5:40 pm

I think earlier references to Keymer Junction should be Copyhold Junction.

There was a collision north of Keymer Junction on 23rd December 1899 (report here: http://www.railwaysarchive.co.uk/docume ... on1899.pdf). That is too early for track circuits to be relevant.

There was a collision at Copyhold Junction north of Haywards Heath on 16th December 1972. The report (at http://www.railwaysarchive.co.uk/docume ... ld1972.pdf). Paragraph 32 of this report considers the benefits of separate berth and overlap track circuits at the signals protecting the converging junction. Although separate circuits would have prevented this accident, the Inspector concludes that there is not "an argument for the general readoption of separate berth and overlap track circuits with the additional costs involved."

I note "readoption" there, not just "adoption".
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Re: Combined approach and overlap track circuits

Unread postby Craig » Thu Oct 26, 2017 7:31 pm

Fast Line Floyd wrote:
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.


Thanks, do you have a reference number for the standard which details this by any chance?
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Re: Combined approach and overlap track circuits

Unread postby John Hinson » Fri Oct 27, 2017 7:59 am

JG Morgan wrote:I think earlier references to Keymer Junction should be Copyhold Junction.

Thanks for that.

I think I am probably getting two incidents mixed up. The one at Keymer would have been a little later than Copyhold but was probably only a near miss because I can't find any accident report for it. But it was much talked about at the time - mid-seventies I would say.

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Re: Combined approach and overlap track circuits

Unread postby davidwoodcock » Fri Oct 27, 2017 11:30 am

I was a regular Brighton line commuter from the mid-1970s and I knew of virtually everything that went on along the route but have never heard of a near miss at Keymer Junction - which I would have thought unlikely anyway because of the layout of the signalling at the junction (with the almost adjacent level crossing on the "branch").

However I was personally involved in a very serious near-miss at East Croydon in the mid-1980s, when my train came to a stand less than a metre short of the collision point with a train going the other way and which I now suspect that combined approach/overlap t/cs may have contributed to causing, and I do know for a fact that that incident was hushed up (apparently with the connivance of the DfT because of the potential cost implications of putting a basic interlocking problem right - the area was about to be resignalled anyway) - which suggests to me that incidents could be hushed up if it was considered expedient to do so.
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Re: Combined approach and overlap track circuits

Unread postby Craig » Fri Oct 27, 2017 3:51 pm

Fast Line Floyd wrote:
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.


Current NR/L2/SIG/30009/E450 [ Issue: 1 ] Signalling Principles Handbook – Overlap Proving states different and our latest workstation installations comply with that, at certain locations we can have a facing set of points within an overlap flashing out of correspondence due to a failure with the signal the overlap applies to showing a single yellow aspect, a derogation to standard GK/RT0060 Interlocking Principles is referenced in Network Rail Standards bulletin board 125 which states,
Following a risk assessment which considered the likelihood of harm from not detecting facing points in the
overlap a safety justification was prepared; a derogation was granted by RSSB against GK/RT/0060
(09/096/DGN and Tracker 6312). The SPH E450 ‘Overlap Proving’ was issued which contained the following
requirement:
“points within the overlap shall not normally be detected”.
The derogation can be found at https://www.rssb.co.uk/rgs/deviations/09-096-dgn.pdf

We actually reported it as a fault with the simulator for the project when we saw it happening, 25 years of signalling and operational experience told me it didn't look right!!, the reply was that the standards had changed and after some digging the above was found!

Again, sorry for deviating from the original topic!
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Re: Combined approach and overlap track circuits

Unread postby Fast Line Floyd » Fri Oct 27, 2017 7:10 pm

Craig wrote:
Fast Line Floyd wrote:
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.


Current NR/L2/SIG/30009/E450 [ Issue: 1 ] Signalling Principles Handbook – Overlap Proving states different and our latest workstation installations comply with that, at certain locations we can have a facing set of points within an overlap flashing out of correspondence due to a failure with the signal the overlap applies to showing a single yellow aspect, a derogation to standard GK/RT0060 Interlocking Principles is referenced in Network Rail Standards bulletin board 125 which states,
Following a risk assessment which considered the likelihood of harm from not detecting facing points in the
overlap a safety justification was prepared; a derogation was granted by RSSB against GK/RT/0060
(09/096/DGN and Tracker 6312). The SPH E450 ‘Overlap Proving’ was issued which contained the following
requirement:
“points within the overlap shall not normally be detected”.
The derogation can be found at https://www.rssb.co.uk/rgs/deviations/09-096-dgn.pdf

We actually reported it as a fault with the simulator for the project when we saw it happening, 25 years of signalling and operational experience told me it didn't look right!!, the reply was that the standards had changed and after some digging the above was found!

Again, sorry for deviating from the original topic!

GK/RT0060 was withdrawn on 7/12/2013 and only the block system requirements and ground frame requirements (the latter written by myself) were kept as Railway Group Standards the rest was by default devolved to NR therefore derogations are internal to NR since that date.
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