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Axxx1
Mar 15, 2013 21:15:31 GMT
Post by Deleted on Mar 15, 2013 21:15:31 GMT
Can anyone tell me the actual purpose of the Auto signals on the Picc that are numbered the same as the signal in rear but with a 1 added (eg A6541 after A654). They don't have sufficient sighting time but are proved clear by the signal in rear being clear. I've heard them referred to as 'round the bend' and 'sighting' signals but haven't been able to get an explanation of their purpose. Thanks in advance!
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Axxx1
Mar 15, 2013 22:14:47 GMT
Post by Nortube on Mar 15, 2013 22:14:47 GMT
I've just checked the Northern line Signal Reference book and there are 22 signals like that. Three digit signals have 1'1 added after the number, or even a '2' in the case of the sequence A717, A7171, A7172 between South Wimbledon and Morden SB. On the Charing Cross branch where the signals have two digits, then '11' is added. E.g. A5511 follows A55 (Waterloo - Kennington SB).
As far as I know, this was purely when additional signals were added (for whatever reason), at some time after the original signals were installed. Presumably they didn't want to renumber a signal run just so that they could fit an additional signal number in in sequence, and this was the result. Without looking at any of the signals track circuits, as far as I'm aware they (or at least most of them) work in exactly the same way as any other auto signal regarding clearance etc.
It is quite likely that some were installed because of sighting problems. It's also possible that some were added in order to allow more trains to be signalled in an area.
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Axxx1
Mar 15, 2013 22:36:42 GMT
Post by Deleted on Mar 15, 2013 22:36:42 GMT
The ones on the Picc are mostly on bends and have no repeater, so as such there would be no way of stopping at one in time if it was at danger. There are only a handful of places where you can observe their operation, but one example is at Bounds Green on the east, where the advance starter has the same number as the starter but with a '1' suffix, and the two clear at the same time. So assuming they all work like this, they don't afford any extra line capacity.
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Axxx1
Mar 15, 2013 22:45:55 GMT
Post by Nortube on Mar 15, 2013 22:45:55 GMT
A similar thing happens on the CX branch, although it may only be in the controlled areas. There's a name for it, but I can't remember what it is. I suspect that A6541 and A8911 work in different ways. It sounds that A8911 doesn't have it's own signalling section and effectively works as a co-acting signal with A891. I'll have a look at the diagram and see, but a Tech would be able to give you the exact explanation.
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Axxx1
Mar 15, 2013 23:01:54 GMT
Post by Nortube on Mar 15, 2013 23:01:54 GMT
I've looked at the diagram and A8911 doesn't have its own track circuit number and so it's quite likely that it works in conjunction with A891, only going to green when A891's section is clear. I said it effectively worked like a co-acting signal. Of course this isn't strictly true because presumably A891 would go to red as soon as the first pair of wheels went over it's block joint as the train started to depart the platform, whereas A8911 would remain green until the first pair of wheels went over its blockjoint.
A891 and A8911 will not go to green until the rear of the train in front has cleared the blockjoint of RPJ1 - about when the front of the train is 2 - 3 cars out of the tunnel.
A6541 has its own track circuit number separate from A654 although that doesn't mean it acts completely independently as track circuit number 6541 could, in fact, be part of A654's signal section - similar to the arrangement with A891 and A8911
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Axxx1
Mar 16, 2013 0:42:03 GMT
Post by Nortube on Mar 16, 2013 0:42:03 GMT
The three diagrams below show the operation of the signals in the Euston NB CX area. The starter is A98, the next signal is A9811. A9811 doesn't have its own track circuit number. It would appear to operate in the same way that I assume A8911 does at Bounds Green. 1 2 3 1 - Euston CX NB description 2 - Start of sequence, train in platform 3 - End of sequence, the train has cleared the sections of A98 and A9811
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Axxx1
Mar 17, 2013 0:39:21 GMT
Post by Deleted on Mar 17, 2013 0:39:21 GMT
Thanks for the thoughts/info. They must serve a specific purpose otherwise they're just an extra set of signal heads and trainstops to maintain. I wonder if they're to provide additional mitigation against collision in the event of a driver doing a hit & run (knowingly or not), although with SCAT nowadays the risk is very minimal.
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Axxx1
Mar 17, 2013 1:46:09 GMT
Post by Nortube on Mar 17, 2013 1:46:09 GMT
That was the reason I was originally given when I drew the diagrams. There have been several SPADs over the years that have ended in a crash because the driver was applying the rule too fast, some resulting in fatality(s). I suppose that, by putting the additional signals in, it's another signal for the driver to see at red and which would trip the train.
As you say, SCAT has helped to reduce the speed and greatly reduced the risk. However some people seem to misunderstand SCAT and think that they can now go at up to 10mph when applying the rule and not, which was a suitable description for the speed in the past, "a slow walking pace"
Some station starters can have very short overlaps (perhaps as little as a hundred feet). The last home signal can clear as soon as the last pair of wheels of the departing train has cleared the blockjoint at the end of the overlap, allowing the following train in to the platform. Meanwhile, if the (automatic) starter is still on, the train in front could be anywhere from just past the end of the overlap to the end of the starter's section (the point at when the starter can go to green). In the case of A98 on the diagram, it would be red if any part of Track circuits 98A to 98c were occupied. Therefore, if the starter is red, the train in front could be as little as 100 feet away.
Obviously, if the end of the train can still be seen, it's a reminder to the driver applying the rule that there is a train ahead. Where the train is stopped round a bend and out of sight, then there's always a very remote chance that if the following train was moving too fast, the driver might not be able to stop in time.
Adding the additional signal can serve two purposes: It gives the driver of the following train a red light, which is a reminder to keep their speed down, and it also forces the train to stop. To proceed, the driver will then need to apply the rule at that signal if it is still red. By the time the driver has reached that signal, they may then have a clear view of the road (and any train) ahead.
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Axxx1
Mar 17, 2013 2:05:25 GMT
Post by Nortube on Mar 17, 2013 2:05:25 GMT
Stratford came to mind as I was typing the last post, so I looked up the crash report of 1953: www.railwaysarchive.co.uk/documents/MoT_Stratford1953.pdfIt's possible that these signals were introduced following that. The crash was on a bend and apparently there was only about four car lengths sighting distance of the train in front. The train that was applying the rule hit the one in front at 20mph and twelve people were killed. There was also a crash in similar circumstances at almost the same spot when a member of staff was killed and three members of staff (in the cab) were seriously injured. The train was empty and was doing 10mph: www.railwaysarchive.co.uk/documents/MoT_Stratford1946.pdfApparently, when the driver was asked why he was going so fast, he said that he didn't expect to find the train so near Stratford.
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Axxx1
Mar 17, 2013 13:33:16 GMT
Post by hellocontrol on Mar 17, 2013 13:33:16 GMT
The signals were known as Stratfords following the accident there in the 1950s, I think there may be some oddities but that is London Transport for you.
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Post by Nortube on Mar 17, 2013 13:36:57 GMT
Thanks
Oddities on London Transport? Never!
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Axxx1
Mar 17, 2013 13:39:47 GMT
Post by hellocontrol on Mar 17, 2013 13:39:47 GMT
Ha Ha
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Axxx1
Mar 17, 2013 17:57:56 GMT
Post by railtechnician on Mar 17, 2013 17:57:56 GMT
Can anyone tell me the actual purpose of the Auto signals on the Picc that are numbered the same as the signal in rear but with a 1 added (eg A6541 after A654). They don't have sufficient sighting time but are proved clear by the signal in rear being clear. I've heard them referred to as 'round the bend' and 'sighting' signals but haven't been able to get an explanation of their purpose. Thanks in advance! "are proved clear by the signal in rear being clear" If that were so then a train passing A654 would be stamping on the anchors because as it passed A654 and dropped on its replacing track it would also replace A6541. Auto signals are proved clear by the tracks 'ahead' being clear. AFAIR these signals are standard auto signals interposed between two existing original signals, in this case A6561 is between A656 and A654 and A6541 is between A654 and A652, DOT being from high to low east to west on the westbound road between Earls Court and Barons Court.
A similar arrangement is between Hounslow West and Hatton Cross westbound where A408 is followed by A4081 and A406 and similar on the eastbound where A409 is followed by A4091 and A411A, A411B approaching Hounslow West.
These are additional signals on curves and that's why they are called 'round the benders'. While it may seem fair to assert that the additional signals have insufficient sighting time themselves is misleading, what is important is that they are providing additional sighting time for a signal ahead that cannot be seen due to obstruction, i.e. the curve.
As for sighting time of the additional signals it is worth making the point that sighting time is taken into account, for instance A4081 is preceeded by R4081/1, R4081/2 and R4081/3 while A4091 is preceeded by R4091 411A/1 and followed by R411A/2 , this cut & cover curve being long and sweeping, line speed through this area being relatively high as I recall. On the other hand the curve between Earls Court and Baron's Court is less severe and less obstructive and the line speed is less.
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Axxx1
Mar 18, 2013 0:13:22 GMT
Post by Deleted on Mar 18, 2013 0:13:22 GMT
"are proved clear by the signal in rear being clear" If that were so then a train passing A654 would be stamping on the anchors because as it passed A654 and dropped on its replacing track it would also replace A6541. Auto signals are proved clear by the tracks 'ahead' being clear. I suspect that what I've said may mean something different in engineering/maintenance circles. By "proved clear" I mean that as a Train Operator, I know that A654 will not clear unless A6541 is clear. I made no reference to the manner in which they would return to danger (which of course would be individually, when a train passes their respective blockjoints). These are additional signals on curves and that's why they are called 'round the benders'. While it may seem fair to assert that the additional signals have insufficient sighting time themselves is misleading, what is important is that they are providing additional sighting time for a signal ahead that cannot be seen due to obstruction, i.e. the curve. It's not misleading at all - there are examples in the South Ken Bends located on the right hand side of the tunnel on a right hand curve which are visible for about two seconds when travelling at the signed speed restriction of 20mph. As to providing additional sighting time for signals in advance; quite simply, they don't. That's what we have repeaters for. As for sighting time of the additional signals it is worth making the point that sighting time is taken into account, for instance A4081 is preceeded by R4081/1, R4081/2 and R4081/3 while A4091 is preceeded by R 4091 411A /1 and followed by R411 A/2 , this cut & cover curve being long and sweeping, line speed through this area being relatively high as I recall. On the other hand the curve between Earls Court and Baron's Court is less severe and less obstructive and the line speed is less. A4081 and A4091 are the only two signals of this type that I have noticed have repeaters. However, R4091.411a/1 is located far too close to A4091 to provide enough stopping distance were A4091 to be at danger, so why the repeater is numbered as such I don't know. I'm also 99% certain that R408.4081/1 clears at the same time as A408, but I'll double check this next time I follow another train through the area.
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Axxx1
Mar 18, 2013 3:22:19 GMT
Post by railtechnician on Mar 18, 2013 3:22:19 GMT
AF
You have eliminated the ambiguity/error in your original comment in your further comment "I know that A654 will not clear unless A6541 is clear" which you may have meant but did not say originally, appearing in fact to suggest the opposite.
A6541 is not dependent upon A654, hence my comment. The limit of control of each of these signals will extend from the replacing track of each signal up to and including the overlap of A652. This is why both signals will be seen to clear simultaneously even though they have independent selection circuitry.
My understanding has always been that strictly speaking the signalling is, or would be, adequate without the 'round the benders' when using the formula to calculate the stopping distance of a train running at maximum permitted speed and spadding. However, as we are aware this has not prevented accidents in the past when applying the rule and exceeding 'walking pace'. You might say that the additional signals are part time policemen because they will only come into play if a spad has occured at the previous signal, otherwise the motorman would normally only ever see them as clear. Hence my comment regarding sighting time of those signals. The point is that in the case of say A4091 you cannot reach it at danger unless you have spadded at A409 because the same tracks beyond the replacing track blockjoint of A4091 are required unoccupied to clear both A409 and A4091. In both theory and practice you'll never approach a 'round the bend' signal at danger at more than 'walking pace' with SCAT enabled trains and never should in practice even if there is no SCAT but if so you'll still be going slowly enough to be stopped before reaching any occupied track ahead. In your South Kensington example you would not be approaching A6811, displaying a red aspect, at 20mph !
Never forget that it's 'green for danger' as one of my old signal installation supervisors used to remind me when I was a young wireman! It is good practice to try and see the obverse of a situation particularly when it comes to safety signalling.
Hopefully this is a clearer explanation of the points I made rather hurriedly and somewhat poorly earlier.
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