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Post by A60stock on Aug 10, 2015 21:36:12 GMT
with an increase to 36tph, would it not be a wise idea to build a third platform at brixton as it seems two platforms will only just about suffice!
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Post by superteacher on Aug 10, 2015 21:55:24 GMT
It would cost a fortune! You'd have to bore out a third tunnel, link it to the other platforms, dig out a new crossover tunnel, change the track layout . . . The list goes on!
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Post by Deleted on Aug 10, 2015 21:57:07 GMT
Well if you're paying for it... Also be a bit careful, more platforms doesn't necessarily help. Three platforms allow for longer layover times, of course, but they also increase the number of (often low speed) crossing moves required which can easily gum up the area. What would probably be better (again, if you're offering to pay for it), is a reversing loop. Either that or some kind of (very expensive) flying junction. Mind you, getting the gradients sensible might well be impossible without building a whole new station.
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Post by spsmiler on Aug 10, 2015 23:47:50 GMT
Wasn't it once mooted that a reversing loop should be built, perhaps even with a new station?
The loop of course would be unidirectional, so the station could easily have three platforms (ahead, left, right) which surely would not involve crossing moves that gum up the area? Indeed, it could benefit the service as always a train would be ready to depart at the correct time even when the next arrival is slightly late / delayed, etc.
Would it happen? I suppose that finance would be the issue.
Simon
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Post by Tubeboy on Aug 11, 2015 8:17:04 GMT
Wasn't it once mooted that a reversing loop should be built, perhaps even with a new station? The loop of course would be unidirectional, so the station could easily have three platforms (ahead, left, right) which surely would not involve crossing moves that gum up the area? Indeed, it could benefit the service as always a train would be ready to depart at the correct time even when the next arrival is slightly late / delayed, etc. Would it happen? I suppose that finance would be the issue. Simon Yes, a loop line to Herne Hill.
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Post by Deleted on Aug 12, 2015 9:25:23 GMT
What is needed is an extension further south, so that those people in the procession of buses clogging up Brixton Hill and Streatham High Road can instead get directly onto the tube at a station near their homes. Of course, this opinion may be slightly biased given that I am often one of those people :-)
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Post by Deleted on Aug 12, 2015 11:26:35 GMT
I agree that, if you're spending a lot of money and causing serious disruption for quite some time, you might as well extend the line further south.
That extension will probably happen at some point, but not until the Victoria line is less crowded between Brixton and Victoria. Perhaps after Crossrail 2 is operational...
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Post by Deleted on Aug 23, 2015 9:51:39 GMT
with an increase to 36tph, would it not be a wise idea to build a third platform at brixton as it seems two platforms will only just about suffice! I submitted a Innovation idea to TfL for a grade separated approach for two track termini. The response claimed that 36tph could be achieved in Brixton style termini using recent improvements (improved train performance, signalling, and RTD lights coming on well before the actual departure time to speed up dispatch). Due to dwell times on the line, more than 36tph is doubtful, but if more than 36tph were required (which would require shorter RORI at busy stations): A third platform at Brixton with grade separated approach would be hugely expensive, and also confuse passengers - as you would have to go down separate escalators to get the 3rd platform, only to potentially miss a train, and then have to wait for two trains to depart from the existing platforms first. Moscow Metro and Paris Metro reverse around 40tph by reversing in sidings beyond the platform, however this would only work if trains are not tipped out. Likewise a non-passenger loop would work, though tight curve radii are avoided these days (due to wear and tear), as are turned trains, plus trains couldn't be tipped out. A passenger loop (and extension) would also allow for increased capacity at high expense (as it would require a new station), but would have the turned trains issue (though this is common on plenty of other lines), and large curve radii would increase the amount of tunnelling.
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Post by castlebar on Aug 23, 2015 12:37:17 GMT
@ stephenk
I wish you well with your loop proposal. Until The Picc was extended, LT seemed to have a totally fixed obsession with "handed stock" (inflexible A & D ends etc were preferred), and I wouldn't be surprised if the Kennington Loop goes OOU one day.
It would be exceptionally useful to 'think outside the box' and use loop workings on surface routes. It could solve "Circle" problems, but there was a time when LT would think of "Any option but that one"
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Post by norbitonflyer on Aug 23, 2015 20:16:47 GMT
Until The Picc was extended, LT seemed to have a totally fixed obsession with "handed stock" Actually, the Picc still operates handed stock, but ambidextrous stock had been introduced on the surface lines (except the District Main) and the Northern, and was already planned for the Central, long before the Heathrow T4 extension was built.
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Post by rsdworker on Aug 23, 2015 21:13:32 GMT
some non passegner loops allow trains with passegners on it to loop back to same station - example wheelchair wants exit at one platform but platform lift is out of order so he has to board the train that going back to other side of station via loop so he will have to remain on train for few minutes before moving to platform again - example in world - New york - old city hall station loop which allows passengers to remain on train as train loops back to Brooklyn Bridge-City Hall
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Post by Chris M on Aug 23, 2015 23:34:27 GMT
A passenger loop (and extension) would also allow for increased capacity at high expense (as it would require a new station), but would have the turned trains issue (though this is common on plenty of other lines), and large curve radii would increase the amount of tunnelling. The turned trains issue could be mitigated by building a matching one at the north end of the line. Not cheap though.
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Post by Deleted on Aug 24, 2015 3:59:23 GMT
The big downside, of course, with reversing loops is that they can't offer the recovery time that a proper terminus can, so it would be hard to reliably operate the timetable. A single reversing loop gives you, basically, a semi-circle; with two loops giving you a full circle. And we know that operating services in a circle is quite problematic, hence the Circle line extension to Hammersmith. And with the Victoria line operating such high frequencies, there isn't really anywhere else on the line where you can add dwell time to give you a bit of a buffer. Now, you may be able to mitigate against this by having a reversing loop as well as a proper terminus, however, this would be expensive and would require extremely good ATC to really deliver improvement. The last thing you want is to have a train ready to depart from the terminal platform waiting for a train to come off the loop. It would undo all of your good work. This is good work which may well be in vain anyway, since you quickly start running into problems with average dwell times at stations eating into your headways. Ultimately, there is, of course, an absolute limit to the number of trains you can put through a line. The Victoria line is probably quite close to the limits of what we can do, especially with a two-track tube line, where everything has to run in single file, more or less. Although it does have the advantage of not having any real junctions, excepting the junction between the depot access roads and the main line. Having said that, rumour has it that the CSLR managed 44 tph, but then we know that the CSLR had - shall we say - a different attitude to safety than the one we have today. It is also said that the Northern line once managed 40 tph on an experimental basis, though was that before Moorgate control and such like? In either case, it was deemed unsustainable. @stephenk's comment about the Moscow and Paris Metros managing 40 tph is very interesting though. I genuinely wonder whether we can deliver that reliably in London. A terminal reversing siding, of course, has pros and cons, as we were discussing in a thread about Wood Green. There must be little margin for error and little recovery time. Also, one thing that does come to mind: Imagine if a train broke down in the departure platform with a train already in the siding and it hadn't been tipped out! And, obviously, if you do have tipping out, you can't run 40 tph. So, we return to the possibility of a third platform, which would give better recovery time, but would be very expensive and - without @stephenk's even more expensive grade separation - would add conflicting moves and may reduce capacity. And so ultimately, once you pass a certain frequency, the problem becomes clear. Any more improvement - if it's even possible - is so expensive, that you start to wonder whether you mightn't be better off building a new line. With improvements to the Northern line and the various Crossrail's, maybe the Vic is finished?
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Post by Deleted on Aug 24, 2015 8:52:07 GMT
The big downside, of course, with reversing loops is that they can't offer the recovery time that a proper terminus can, so it would be hard to reliably operate the timetable. Incorrect. A passenger loop with two platforms used alternately would allow for up to 155secs dwell time (with 45sec RORI), and 125secs recovery time @ 30 sec minimum dwell time. That is a lot of recovery time! A non-passenger loop would allow for recovery time based on how many trains are queued in the loop (a la Kennington).
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Post by Chris M on Aug 24, 2015 9:04:17 GMT
At some point, the limit to throughput becomes the length of time it takes people to board and alight the train at each station. Good train design can help with this, but there is a limit to what can be done without the ratio of doors to walls being reversed. An expensive solution to this is having two platforms per direction per station, so that the second train can pull into the platform before the first train has fully left (I think this is the layout London Bridge mainline's through platforms will have when the upgrade is complete).
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Post by Deleted on Aug 24, 2015 11:57:09 GMT
At some point, the limit to throughput becomes the length of time it takes people to board and alight the train at each station. Good train design can help with this, but there is a limit to what can be done without the ratio of doors to walls being reversed. An expensive solution to this is having two platforms per direction per station, so that the second train can pull into the platform before the first train has fully left (I think this is the layout London Bridge mainline's through platforms will have when the upgrade is complete). Once a line is reaching around 36tph with modern signalling, it is time to build a whole new line. The advantages of a complementary route, the new journey opportunities it creates, and doubling in capacity is far in excess of spending $$$ on trying to squeeze in a few more tph with extra platforms, terminus improvements, etc.
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Post by Deleted on Aug 24, 2015 16:34:25 GMT
The big downside, of course, with reversing loops is that they can't offer the recovery time that a proper terminus can, so it would be hard to reliably operate the timetable. Incorrect. A passenger loop with two platforms used alternately would allow for up to 155secs dwell time (with 45sec RORI), and 125secs recovery time @ 30 sec minimum dwell time. That is a lot of recovery time! A non-passenger loop would allow for recovery time based on how many trains are queued in the loop (a la Kennington). Very fair point that, I hadn't considered using the platforms alternately. The other thing that I'd obviously been forgetting is that the current 2 platform layout at Brixton offers so little recovery time anyway and that, if you want to run 36 tph, recovery time is always going to be an issue. Unless you have branches. But then you have the problem of managing traffic through junctions and of, say, 34 tph through the core, but only 17 tph on the branches.
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Post by spsmiler on Aug 24, 2015 20:59:21 GMT
The big downside, of course, with reversing loops is that they can't offer the recovery time that a proper terminus can, so it would be hard to reliably operate the timetable. Incorrect. A passenger loop with two platforms used alternately would allow for up to 155secs dwell time (with 45sec RORI), and 125secs recovery time @ 30 sec minimum dwell time. That is a lot of recovery time! A non-passenger loop would allow for recovery time based on how many trains are queued in the loop (a la Kennington). I am not into scheduling times but if a loop is built then the single station should have at least three tracks, with trains always entering at one end and leaving at the other end. Not only would this give some recovery time but it would help smooth out delays if a train should be delayed a few minutes because of a longer than expected station stop. Simon
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Post by superteacher on Aug 24, 2015 21:04:31 GMT
Of course, the higher the frequency, the lower the loading thus reducing dwell time. It isn't quite that simple, I know, but that's the theory at least,
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Post by Deleted on Aug 24, 2015 21:09:34 GMT
Incorrect. A passenger loop with two platforms used alternately would allow for up to 155secs dwell time (with 45sec RORI), and 125secs recovery time @ 30 sec minimum dwell time. That is a lot of recovery time! A non-passenger loop would allow for recovery time based on how many trains are queued in the loop (a la Kennington). I am not into scheduling times but if a loop is built then the single station should have at least three tracks, with trains always entering at one end and leaving at the other end. Not only would this give some recovery time but it would help smooth out delays if a train should be delayed a few minutes because of a longer than expected station stop. Simon Three tracks in an underground station would cause customer confusion and massively increase construction costs. As calculated, two platforms in a passenger loop would provide sufficient recovery time - 125 secs at 36tph, instead of approx. 10-15 secs at 36tph at Brixton.
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Post by Chris M on Aug 24, 2015 21:56:31 GMT
That depends how the platforms are organised. A single alighting platform before the loop and two boarding platforms after it should not cause customer confusion if they were signed appropriately.
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Post by Deleted on Aug 25, 2015 9:43:35 GMT
That depends how the platforms are organised. A single alighting platform before the loop and two boarding platforms after it should not cause customer confusion if they were signed appropriately. I don't think that was what was being referred to. A non-passenger loop as you describe would only work if trains are not tipped out in the arrival platform. Again, a 3rd platform is not cost effective as any recovery time can be taken with trains queuing in the loop rather than in very expensive to build extra platforms. Don't forget that if there is more than two platforms (island configuration), the amount of access infrastructure (escalator shafts, lift shafts, emergency staircases) has to effectively double, massively increasing construction costs.
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Post by spsmiler on Aug 25, 2015 22:22:17 GMT
I am not into scheduling times but if a loop is built then the single station should have at least three tracks, with trains always entering at one end and leaving at the other end. Not only would this give some recovery time but it would help smooth out delays if a train should be delayed a few minutes because of a longer than expected station stop. Simon Three tracks in an underground station would cause customer confusion and massively increase construction costs. As calculated, two platforms in a passenger loop would provide sufficient recovery time - 125 secs at 36tph, instead of approx. 10-15 secs at 36tph at Brixton. Well Seven Sisters was given three platforms! As for the confusion, it can't be any worse than at Stratford on the Jubilee Line. I feel sure that the sign on the concourse in front of the three platforms (earlier today) said that the first train out was at platform 13, yet as I was walking along that platform I noticed that it was the train at platform 15. However, that said, it would be easier not having to make such choices, which is possible with just two tracks and an island platform, as per Heathrow Central until the T5 extension was built. Perhaps to reduce conflict at the stations between arriving and departing passengers the two tracks would adopt the Spanish Solution and use outside side platforms for arriving passengers and a central island platform for departing passengers - a variant of this is done at Tower Gateway DLR, albeit only serving a single track at a dead-end terminus. Simon
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Post by Deleted on Aug 26, 2015 10:53:28 GMT
Three tracks in an underground station would cause customer confusion and massively increase construction costs. As calculated, two platforms in a passenger loop would provide sufficient recovery time - 125 secs at 36tph, instead of approx. 10-15 secs at 36tph at Brixton. Well Seven Sisters was given three platforms! As for the confusion, it can't be any worse than at Stratford on the Jubilee Line. I feel sure that the sign on the concourse in front of the three platforms (earlier today) said that the first train out was at platform 13, yet as I was walking along that platform I noticed that it was the train at platform 15. However, that said, it would be easier not having to make such choices, which is possible with just two tracks and an island platform, as per Heathrow Central until the T5 extension was built. Perhaps to reduce conflict at the stations between arriving and departing passengers the two tracks would adopt the Spanish Solution and use outside side platforms for arriving passengers and a central island platform for departing passengers - a variant of this is done at Tower Gateway DLR, albeit only serving a single track at a dead-end terminus. Simon Seven Sisters has three platforms as one is used to tip out trains exiting service - quite a legitimate cost/benefit. Stratford, and LU's collection of other 3 track dead end surface termini do cause passenger confusion as they can just miss a train and then have to transfer platforms. However as these are surface stations, the cost implications of the 3rd platform is much less. As this discussion is referring to underground deep level termini, 3 track termini are a different kettle of fish.
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