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Central London 'PoSA' signalling

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Central London 'PoSA' signalling

Unread postby RobMorel » Sat Mar 10, 2018 2:15 pm

Small video clip of Proceed On Sight Authority signalling in use at London StPancras Low Level earlier this week
https://vimeo.com/258849722

and further reading, the basics,here
https://www.rssb.co.uk/rgs/rulebooks/GE ... ss%203.pdf
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Re: Central London 'PoSA' signalling

Unread postby Ashley Hill » Sat Mar 10, 2018 7:15 pm

Does the PoSA signal stay flashing once the train has passed as in the video or does it stop once the train has cleared the overlap?
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Re: Central London 'PoSA' signalling

Unread postby RobMorel » Sun Mar 11, 2018 9:27 am

PoSA aspect extinguished once train passes overlap
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Re: Central London 'PoSA' signalling

Unread postby StevieG » Sun Mar 11, 2018 3:34 pm

Wonder if that was set up for demo / testing / training purposes, with all train detection sections known to be clear.
Train speed looks to be a little fast if only route set proved with sections not proved clear in a tunnel.
Or perhaps I'm not quite clear on circumstances of the POSA aspect being displayed.
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Re: Central London 'PoSA' signalling

Unread postby RobMorel » Sun Mar 11, 2018 5:27 pm

TPWS Overspeed sensor remains active during PoSA working , Line speed 30mph normally with OSS set at 22 mph, in use due to TC SOWC, Train Stop sensor not active, a few trains 'caught' with the OSS.....
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Re: Central London 'PoSA' signalling

Unread postby JG Morgan » Tue Mar 13, 2018 7:27 am

Interesting - thank you.
That looks like an ordinary train, not demo / training / testing, as there is a good number of what appear to be ordinary passengers on board.

Re StevieG "Train speed looks to be a little fast ..."
From the lack of screeching on the curve (compared with a normal southbound departure) I would say that train is running much slower than normal (as per RobMorel's comment).

One day last week I noticed that the train seemed to trundle more slowly than normal from St Pancras to Farringdon for no obvious reason; it was not close behind another train. I can't remember which day it was - but it would have been about 08:45.
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Re: Central London 'PoSA' signalling

Unread postby John Hinson » Tue Mar 13, 2018 4:51 pm

It may be running slower than normal but I don't call that " stopped or nearly stopped" which is what Rule 2.1 says in a rather lopsided manner.

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Re: Central London 'PoSA' signalling

Unread postby Fast Line Floyd » Wed Mar 14, 2018 6:08 pm

Ashley Hill wrote:Does the PoSA signal stay flashing once the train has passed as in the video or does it stop once the train has cleared the overlap?

Only if either the overlap or the berth track are clear otherwise it cannot tell if the train has passed.
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Re: Central London 'PoSA' signalling

Unread postby DY444 » Sat Mar 17, 2018 12:14 pm

John Hinson wrote:It may be running slower than normal but I don't call that " stopped or nearly stopped" which is what Rule 2.1 says in a rather lopsided manner.

John


Does that still apply when the train is starting away from a station stop under a POSA aspect? (which the one in the video was)
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Re: Central London 'PoSA' signalling

Unread postby Mike Stone » Sat Mar 17, 2018 1:03 pm

I sometimes wonder if modern electric trains taking power are unable to control their starting speed below a certain level - leaving Crewe it often appears that a quick start is followed by the train slowing down.
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Re: Central London 'PoSA' signalling

Unread postby John Hinson » Sat Mar 17, 2018 1:37 pm

DY444 wrote:Does that still apply when the train is starting away from a station stop under a POSA aspect? (which the one in the video was)
That isn't evident to the unitiated (i.e me) as the train is moving at the start of the sequence. I guess it depends on what is meant by "at a signal" in Rule 2.1 but to my eyes starting at that distance is certainly not "at the signal". However, with all the rules and limitations to prevent "ding ding and away" incidents, it is difficult to see how a train can actually find itself at that signal if it previously calls at the station.
It is an issue that also affects conventional signalling. It is easy for a driver to call up from his cab for instructions these days and needing to actually stop at the signal to use the phone is becoming a historic pastime. Maybe current rules cover this situation somewhere (older ones didn't) or maybe it is something that has simply slipped attention because no incidents have arisen.
Mike Stone wrote:I sometimes wonder if modern electric trains taking power are unable to control their starting speed below a certain level - leaving Crewe it often appears that a quick start is followed by the train slowing down.
That's an interesting point, Mike. As far as I know they only have a few defined settings on their controllers so that could be the case. Or what you are experiencing could just be a switch from series to parallel (if that's how they do it these days). Older units certainly used to have a "shunt" position which allowed gentle but easily managed movement.

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Re: Central London 'PoSA' signalling

Unread postby Mike Hodgson » Sat Mar 17, 2018 2:24 pm

John Hinson wrote:It may be running slower than normal but I don't call that " stopped or nearly stopped" which is what Rule 2.1 says in a rather lopsided manner.

John


The expression "stopped or nearly stopped" strikes me as a hangover from the old rulebook's stipulations to the effect that you don't clear subsidiary signals until you see the train was fully under control, which was more directed at the signaller anyway. Rule 2.3 is aimed at the driver and I agree it's rather lopsided. Rule 7 does explicitly addresses it to the signaller however.

Although 2.1 says you should have been (nearly) stationary, it doesn't say you need to continue to go dead slow - it almost suggests that once you've got your PoSA it's perfectly OK to accelerate likely billyo. Given the acceleration of modern units, the speed at which you proceed is now much more of an issue than in steam days. What does limit the extent that you can accelerate from that (nearly) stop is rule 2.3, which requires you to "proceed at caution". I am rather surprised that in a document with this title I don't find any definition of "at caution", "on sight" nor of any requirement to be able to stop short of obstructions. This may well all be covered by other rule books, but I would have thought it appropriate to include these points in this book's defintions (Rule 1) or at least a pointer to some other relevant documents. Railway rule books don't usually rely quite that heavily on using common sense. No maxiumum speed is stipulated although that may be implicit with ERTMS where applicable. We can't rely on TPWS as its failure might be the reason we are using PoSA.
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Re: Central London 'PoSA' signalling

Unread postby John Hinson » Sat Mar 17, 2018 4:15 pm

I agree. But it is really a case of passing a failed signal at danger but being advised by indication rather than by signalman (although he may be involved). I think the comparison should be made with that rather than subsidiary signals despite the visual similarity. In that instance, trains always had to be at a complete stand in order to receive instructions by phone or verbally. I actually think the use of this specific design of signal is a poor choice for the purpose.

There are two kinds of proceeding at caution:
  1. Where a train proceeds slowly looking for something like a reported cow on the line, for instance
  2. When a driver is allowed into a section which could be occupied - anything from an unexpectedly occupied track circuit to time-interval working (although I suspect the latter is history now). But the point is that in this instance the driver must proceed at a speed at which he can stop short of any obstruction. Usually those words are used with the driver, or should be, because nobody can be sure there isn't a train or vehicle in the section.
I don't know how clearly the above two are defined in the current rule book but it may be that what is there is deemed good enough to cover the type of signalling under discussion. What I do know from throughout my railway career is that drivers often put more faith in the signalmen than they should. Perhaps the most hair-raising experience of that I had was as a passenger on the Southern Region where I heard the signalman giving the driver verbal instructions for time interval working, and then found myself hurtling through the section at line speed! Some things are better not known . . .

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Re: Central London 'PoSA' signalling

Unread postby Ashley Hill » Sat Mar 17, 2018 4:30 pm

Sorry chaps,not overly familiar with PoSA signals. It seems to be intimated that the clearance of the PoSA signal gives authority to pass the signal at danger and proceed at caution even though the line ahead is known to be clear. So the only difference between this and an ordinary position light is that the flashing aspect indicates the red is being passed because of a fault rather than proceed as far as line is clear like a normal position light.
Is this line DOO operated? If so no guard to have a ding ding and away SPAD.
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Re: Central London 'PoSA' signalling

Unread postby Fast Line Floyd » Sun Mar 18, 2018 12:34 pm

Ashley Hill wrote:Sorry chaps,not overly familiar with PoSA signals. It seems to be intimated that the clearance of the PoSA signal gives authority to pass the signal at danger and proceed at caution even though the line ahead is known to be clear. So the only difference between this and an ordinary position light is that the flashing aspect indicates the red is being passed because of a fault rather than proceed as far as line is clear like a normal position light.
Is this line DOO operated? If so no guard to have a ding ding and away SPAD.

Quite so, the technical limit of the movement is 'as far as the line is clear or to the next stop signal whichever is the nearer'.
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