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Post by smasher on Jan 17, 2009 12:25:15 GMT
I've read in the Traffic Circular that this signal is being removed from start of traffic on Sunday. It doesn't state the benefits or the reasons why this is being done. Just wondered if anyone knew why?
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Colin
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Post by Colin on Jan 17, 2009 20:49:21 GMT
It does state the reasons in the traffic circular!  |
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Post by Chris M on Jan 17, 2009 20:51:16 GMT
That sounds to me like it will reduce the capacity of the area by 1 train? If so (and I am not unlikely wrong) that doesn't seem like a good thing on the face of it.
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Post by suncloud on Jan 18, 2009 1:04:47 GMT
A 'benefit' may be that a held train will be at a platform rather than in tunnel.
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Post by chrish on Jan 18, 2009 1:09:38 GMT
From my point of view this is a good thing. Obviously it's good that one fewer trains get held in tunnels, but I have on at least 2 occasions come across A167 as the first signal at danger. If a train is coming in the opposite direction at the wrong time, you have no choice but to use emergency brakes to ensure you stop before it!
Scared the living daylights out of me when I found that out! A repeater would also have worked for this, but I am sure there is some logic to removing the signal instead!
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mrfs42
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Post by mrfs42 on Jan 18, 2009 1:20:04 GMT
Surely this is part of the ongoing programme of chasing up 'compromised' overlaps [1]; like the other parts of the Circle have had controls extended on some signals - doesn't the OR drop down towards the soon-to-vanish signal?
[1] as in changes in the formula, making them even safer.
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Post by Deleted on Jan 18, 2009 11:39:50 GMT
The 30mph TSR is in place because of the overlap on that signal. That should come out too then.
I've also had to cosh it on that bend a few times too.
It will knacker the service when trains go through under rule though as it did when they took out half the sticks in the Wood Lane area.
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Post by tubeprune on Jan 18, 2009 14:09:10 GMT
So now the starter at Notting Hill won't clear until the train ahead is clear of the Bayswater starter overlap. This means that the whole dwell plus the run out time is inserted into the throughput. This will widen the possible headway by about one minute.
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Post by Deleted on Jan 18, 2009 14:58:01 GMT
The notification I received this morning also states the starter at Notting Hill Gate will be approach cleared as part of the modifications, which presumably means a slower approach, I always approach a station with a red starter at a reduced speed. I'm not sure I can see the point of all this, surely the Bayswater Home should just have been moved so it was a full speed overlap from the station ?? Preferably with a repeater, this signal also makes me jump a mile if it is red owing to the sighting issues !! Seems like another Elm Park / Becontree / West Ham jobbie to me, far cheaper easier to make a signal redundant and link to the signal in rear, than to move it where it should be .... and disregard the capacity issue pretending that it won't matter because trains always run to their booked timings !! ;D ;D ;D * edit * with a further thought - of course with the introduction of the "Tea Cup" service there is very likely going to be queues of Edgware Road reversers at these stations that will need holding in platforms and a cynic may think this capacity reduction will lend it's self to justifying curtailment of more or all of the Wimbledon - Edgware Road service at High Street !!!!  |
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Post by citysig on Jan 18, 2009 23:52:51 GMT
The signal has gone from today, and I had 2 or 3 drivers who called up having "missed" it and not realised it was gone until reaching Bayswater and suddenly thought "hang on what's happened to that signal."
There was no additional blocking back - Ok we were running a slightly reduced headway today. But I had a thought today that the area is now the same as the Inner Rail. Despite there being a signal between Bayswater and Notting Hill Gate, the Bayswater starter does not clear until the train ahead leaves NHG.
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Tom
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Post by Tom on Jan 19, 2009 8:41:13 GMT
I'm not sure I can see the point of all this, surely the Bayswater Home should just have been moved so it was a full speed overlap from the station ?? It is for compromised overlap mitigation. If it were to be moved to such a position it would no longer provide any headway benefit - hence there is no point in retaining it. The same with all the signals in the Wood Lane area, under normal circumstances they provide no benefit for the cost of retaining them. And we don't put signals down for the benefit of speeding things up when running by procedure - otherwise there would be a signal for every track circuit. A number of options were tabled but the line management rejected all but this one. |
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Post by Deleted on Jan 19, 2009 9:27:15 GMT
From my point of view this is a good thing. If a train is coming in the opposite direction at the wrong time, you have no choice but to use emergency brakes to ensure you stop before it! Scared the living daylights out of me when I found that out! Phew, I thought this only happened to me, but I can now reduce the number of fresh pairs of underpants I carry by one! |
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Post by Deleted on Jan 20, 2009 20:24:52 GMT
If it were to be moved to such a position it would no longer provide any headway benefit - hence there is no point in retaining it. I don't quite follow the logic ... if a train was to be held at a correctly positioned Bayswater home signal, when a train at Bayswater moves off, that train will then be much quicker to next arrive at Bayswater and not be blocking up track circuits further back ... so I'm unclear why there is no headway benefit ?? Whilst this should probably be in it's own thread, but on the subject of compromised overlaps, i notice a fair bit of the eastbound track between Barking and Upminster has been replaced as well as the Westbound. This includes a chunk from Dagenham East to Elm Park, but stopping short of the section that contains the compromised overlap signal (Elm Park outer home) Question is, if this bit of track is to be replaced next, is the opportunity to be taken to resite the Elm park outer home ? |
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Post by Tom on Jan 20, 2009 22:24:55 GMT
Without looking at the scale plans I can't say for definite, but it could be that a correctly positioned signal will be so close to the previous station's starter that a train detained at it will still be in the platform.
I'll have a look at the scale plan and the high level designs and try to work it out.
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Colin
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Post by Colin on Jan 21, 2009 9:56:49 GMT
Whilst this should probably be in it's own thread, but on the subject of compromised overlaps, i notice a fair bit of the eastbound track between Barking and Upminster has been replaced as well as the Westbound. This includes a chunk from Dagenham East to Elm Park, but stopping short of the section that contains the compromised overlap signal (Elm Park outer home) Question is, if this bit of track is to be replaced next, is the opportunity to be taken to resite the Elm park outer home ? Why would 'they' continue to replace the track towards Elm Park [eastbound]? It is already flat bottom and was a only done a few years ago! |
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Post by Ben on Jan 21, 2009 11:27:23 GMT
Forgive my ignorance, but is there much practical (not technical) difference between a signal that has a speed restriction applied to its entire covered area, and one that clears only when a restriction on speed is maintained? I ask only because I cant see much of a different upshot between signals close together with a TSR (or even a perminant one), and signals close together that clear only for a low speed (ie Speed controlled signalling). If the result is similar between the two, then surely removing the extra signal (though widening the headway) results in a quicker journay time between the two stations (upon departure) on account of the removal of the TSR? Now if thats the case, it seems in direct contadiction to the similar thread about Speed controlled signalling earlier, which TP commented on. I'm very confused here; how can the same principals be good in one case yet not in another which shares the same endpoint? Or am I being very dense   |
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Colin
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Post by Colin on Jan 21, 2009 11:51:06 GMT
well......
More signals = improved headways. So if you have less signals, the opposite would be true.
Speed controlled signalling and TSR's (Temporary Speed Limit) for compromised overlaps are two entirely separate things.
Speed controlled signalling is where a given signal will only clear if the train's speed is low enough for that particular signal - a good example of this would be a draw up signal which protects converging lines (ie Whitechapel or Edgware Road, to name two random examples).
Compromised overlaps are where there is a possibility that if the train were to pass a signal at danger at full line speed, it would not stop [having been tripped] before colliding with train ahead. To mitigate against this possibility, either a lower speed limit would be put in (TSR) or the signal before of the one with the compromised overlap would be joined in circuit such that they clear together, thus pegging the train back at a greater distance (ie, West Ham, Becontree & Elm Park on the eastbound District).
There are many factors which dictate which is the best solution in a given area, not least being cost, but also intensity of service required, line speed, stock characteristics, etc, etc....
Does that help?
As for what they've done at Bayswater - they've basically said the headway is reduced anyway because of the compromised overlap, and the service appears to be fine so we might as well get rid of the compromised overlap and stick with the reduced headway.
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Post by mrfs42 on Jan 21, 2009 12:05:07 GMT
Forgive my ignorance, but is there much practical (not technical) difference between a signal that has a speed restriction applied to its entire covered area, and one that clears only when a restriction on speed is maintained? Yes; a speed controlled signal permits reacceleration of the train, once the signal has stepped-up to a less restrictive aspect. A signal that has a speed restriction applied through its limits of control (your covered area) does not allow reacceleration. There is a difference; when a signalling installation is planned - headways, traffic density, station dwell times and the sighting distances/reaction time of the Motorman are taken into account; together with the relative gradients on the approach to and the overlap beyond a signal also the differing braking characteristics of the trains using the line. If you apply a speed restriction, the carefully calculated positions of the signals are disegarded - OK the difference might only be 11 or 12 seconds (plucking that figure out of the air); but consider the cumulative delay. This is important on metro systems, and has a knock-on effect to the planned transit time of the train after passing through the speed restriction - the 11 or 12 seconds will have to be shaved off the next station stop or the motorman might have to wind it up a bit reducing the leeway for motorman error and increasing (albeit mildly) the SPAD potential. Yes, it does result in a quicker transit time between the two stations but in this case there has been an additional control added to the NHG starter, making it approach controlled. What you gain, you lose. Even if you can now go quicker between NHG to Bayswater, you now need to approach NHG slower - shaving a little bit off the dwell time. You don't want to approach NHG at the old speed and get tripped, do you? No, you're not being dense - there are other factors you might not have been aware of like the reaction time of the Motorman, the braking rate of the train &c. What is important here, is that there *is* a small difference between the two solutions - with metro systems, the small differences add up much more quickly. Does that help? EDIT: It's happened again! I was typing this as Colin posted; hopefully the slightly different answer will be of interest. |
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Post by Deleted on Jan 21, 2009 16:31:11 GMT
The current practice seems to be to make sidings accessible both ways, so I assume trains will also be detrained at Plaistow and run to the siding. This will have better flexability than Plaistow bay road alone.
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Post by Tom on Jan 21, 2009 21:29:43 GMT
That's my understanding as well, from the plans I've seen (and helped draw).
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Post by Tom on Jan 21, 2009 21:38:57 GMT
I don't quite follow the logic ... if a train was to be held at a correctly positioned Bayswater home signal, when a train at Bayswater moves off, that train will then be much quicker to next arrive at Bayswater and not be blocking up track circuits further back ... so I'm unclear why there is no headway benefit ?? Having looked at the high level design, it appears that moving the Bayswater home to a compliant location would have introduced a number of additional problems. Firstly, it may well be that the position of a train when berthed at the compliant position may end up foul of the full speed overlap of the NHG starter - which introduces yet another compromised overlap, which is quite clearly a no-no. Secondly, the amount of work to enable such a move is quite involved; a new block joint would have been needed, extended timbers for the repositioned trainstop, conductor rail gapping, relocation of the appropriate track circuit feeds and relays, and the like for like rewiring of the signalling in its new position. As only one berth has been removed, it was the most cost-effective solution. It also makes the signalling on both roads identical (i.e. starter and nothing else until the next starter). No - it isn't in the Track Replacement Project's scope and if the Elm Park outer home has already been mitigated, the risk is already reduced to a level that is ALARP. It may not do much for the service, but it reduces the risk. For the record, signal engineers don't just remove signals or increase the headway on signals for the fun of it - we do get approval from LU management before we commit to a solution, and sometimes the design has to be made 10 times more complicated to meet their requirements. |
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Post by Deleted on Jan 21, 2009 23:15:02 GMT
No - it isn't in the Track Replacement Project's scope and if the Elm Park outer home has already been mitigated, the risk is already reduced to a level that is ALARP. It may not do much for the service, but it reduces the risk. As Colin has observed I quoted the wrong signal as being in the area of track replacement ! I should have said the one between Upney and Becontree !! I assume your answer remains the same. Thanks for the detailed answers throughout this thread sir. It is a shame, if a signal's position is misplaced by current standards and the risk has been mitigated by a quick fix, because a proper fix is too expensive, that when the opportunity to make the proper fix at minimal cost comes along it isn't taken because the risk has already been mitigated, albeit not in the ideal way !!  If the result is similar between the two, then surely removing the extra signal (though widening the headway) results in a quicker journay time between the two stations (upon departure) on account of the removal of the TSR? Only in the prefect world ... ie. if the track sections ahead are unoccupied. If a train is present in the o/r platform at Bayswater, then previously a following train would be held at the home signal, now it will be held at Notting Hill Gate. When the train at Bayswater o/r departs, a train at the Home signal would reach Bayswater very much quicker than one only now able to depart Notting Hill Gate ! Given the frequency of disrupted working with trains right behind each other, plans that presume upon perfect working are a bit short sighted ! (Given ATO on Sub Surface Lines is now a long way off !!). This is a general comment, Tom's note of moving the Bayswater home then compromising another signal makes sence (but isn't applicable at the District east end) |
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