It's not quite as simple as that. Do they drop their pantographs at Harrow, run on d.c. to Amersham, then put them up again (which requires the least changes, but has them at lower than normal power for a long, fast section) or Moor Park to Amersham (which optimises power use, but means pulling up miles of conductor rail and the Met losing the use of the fasts)?
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electrifying to Aylesbury?
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It's not quite as simple as that. Do they drop their pantographs at Harrow, run on d.c. to Amersham, then put them up again (which requires the least changes, but has them at lower than normal power for a long, fast section) or Moor Park to Amersham (which optimises power use, but means pulling up miles of conductor rail and the Met losing the use of the fasts)?
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Chris125
Established Member
I can't see the benefit of OHLE to Moor Park justifying the cost and loss of flexibility, the speed limit isn't massively high and there's no intermediate stops to make use of the better acceleration.
Chris
joeykins82
Member
I think there was some confusion with the comments above in that there isn't a situation currently where trains specifically swap between OHLE and 4th rail; there are plenty of swaps between OHLE & 3rd rail, and 3rd rail stock do run over 4th rail in service.
Aren't those two linked? Surely, re-electrification would be a good time to increase the speed limit, possibly to the likely maximum of 100?
Trog
Established Member
No it is the other way round with 4th rail stock running on modified 3rd rail track.
Where a false 4th rail is fitted electrically connecting the 4th rail shoes of the 4 rail unit to the traction return running rail of the electrically 3 rail track.
If the problem with running AC and 3rd rail DC trains on the same track is the shared earth.
Where the DC system is of the true LT 4th rail type with the fourth -ve conductor rail isolated from the (AC) traction return running rail. Does the problem go away as the two earths are on seperate and electrically isolated rails? With the AC returning through the traction return rail and the DC returning current through the neggie or 4th rail?
Chris125
Established Member
It's possible, but would the time saving justify the cost? I'd be surprised, especially taking into account the comments about track spacing north of Pinner inflating the cost of OHLE and presumably complicating any transfer of responsibility for the fast lines to NR.
Chris
Possibly not when seen simply from a time-saving point of view, but remember its non-standard. Electrification of the Chiltern Lines would most likely involve new 100 mph stock and a full-on route upgrade with whatever the latest signalling is at the time (just as I would expect any other modern suburban electrifacation to deliver). Being stuck with a 70 mph section after this might not make much of an operational difference, but it would stick out like a sore thumb as a bodge. From a personal point of view, I don't like bodges of this sort, and from a political point of view it makes the route and Network Rail look a bit silly, sticking with an Edwardian system because it's too complicated to change to a modern one (which I know isn't much newer, but has more development potential). The phrase that comes to mind is, "Spoiling the ship for a ha'porth of tar".
There may be a 'work-around' solution, however. After all, if a 5-WES or an Electrostar can make 100 on 3rd rail, there's probably no reason why a Chiltern unit can't do so on 4th rail. It might require a voltage-boost to 850 V, something that works on the SWML.
Chris125
Established Member
I don't know if the NR sections would warrant significantly higher speeds alongside the erection of OHLE, probably not for what amounts to little more than an outer-suburban service, but over LU infrastructure it just wouldn't be justified and i can't see there being any desire from either NR or LU for taking over the fast lines south of Moor Park to install OHLE and/or increase the speed limit.
Chris
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HSTEd
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But it wouldn't be running on "4th rail".
An NR unit would be running on third rail.
There is no reason to spend god knows how much to marginally improve journey times on what is frankly a secondary route (Whatever Chiltern and its fans like to claim with its "loco hauled service" - which will probably be obliterated by HS2).
The only reasonable option is to convert the fourth rail over the Met to third rail with a running rail bonded fourth rail, and then run conventional dual voltage units over it.
Anything else is just throwing money away for no good reason.
Wrong route.
You're describing the via High Wycombe route, not the via Amersham one we're discussing. Put simply, the slow speed and lack of capacity currently cause passengers to drive to the WCML and travel from there, adding to the overloading. Improving the speed and adding longer trains on the Amersham route would not only make that route more attractive for modal shift, but will bring those using the WCML back, relieving a very busy railway.
HSTEd
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Ooops......
Even if the number of passengers travelling in from Aylesbury doubles, would that offset the likely loss of passengers from tearing the Metropolitan line service to pieces to allow you to take away the fast lines?
Surely the better option to improving journey times from Aylesbury would be improvements on the High Wycombe route, perhaps by taking steps to increase the line speed on the branchline to Aylesbury from High Wycombe?
And I am not entirely sure how you plan to make trains longer than S8s.... since they come out at almost 130m long.
You can't get that much longer before you start to run into platform length issues there.
You run into platform length issues with anything longer than 5x23m vehicles (115m) at Rickmansworth, which is one of the reasons Chiltern don't stop there in the peaks. And I seem to remember reading somewhere that running anything over a 6 car Chiltern over the Met needs permission from the LU Line Controller, presumably due to signal overlaps and the like.
joeykins82
Member
As I understand it the aspiration for Chiltern & NR is for the Aylesbury via Amersham route to become an isolated service (with dual-voltage stock designed to work on both LU & NR routes) and for the Chiltern Mainline to take Oxford, Stratford on Avon, Milton Keynes via Aylesbury & Aylesbury Vale Parkway, and Birmingham/Kidderminster services. In that context I don't think that the speed limit on the Harrow to Amersham section is a huge issue for Chiltern's plans, certainly not as big an issue as the single track section between Princes Risborough and Aylesbury or the near total absence of passing loops south of High Wycombe.
The pragmatic solution (as I see it anyway) for electrifying the Amersham route would be to use Harrow-on-the-Hill and Great Missenden as the DC/AC changeover points thus avoiding any possible conflicts with LU.
The pragmatic solution (as I see it anyway) for electrifying the Amersham route would be to use Harrow-on-the-Hill and Great Missenden as the DC/AC changeover points thus avoiding any possible conflicts with LU.
The met management seem to have done a great job of doing just that already!
With a decent rebuild of Ricky for interchange and lengthened platform at HotH then I don't see it as a great problem. With stops at those two passengers will have a much better service, abet having to make an interchange.For Aylesbury, yes. That's going to do a fat lot of good for all the other stations between Aylesbury and Amersham/Ricky.
Well, quite. Lengthening platforms is something Chiltern would be much keener on than LU for exactly that reason. Ricky aside, most of the other stations should be easy enough to extend.
With sole NR possession of the fast lines, investment in signalling and longer trains becomes possible.
apk55
Member
The problem with dual electrification that AC transformers are senstive to even small DC voltage, in the order of volts. This voltage would drive the transformer core into saturation causing high current spikes to be drawn from the power supply.
A DC train which draws several thosand amps creates a voltage drop along the running rails which can be in the order of 10's of volts and easily cause transformer problems. However with LU 4th rail system the return current goes along an isolated rail so there should not be a problem. The electrical equipment on LU trains is tested to ensure it is isolated from the coach body and the equipment could run with either rail earthed. Nominaly the electrical system is floating and this has the advantage that an earth fault on either live rail could be tolerated.
Where 3rd rail and OHL electrifications exist a number of measures would be required to reduce the problem of imposing a DC voltage on the AC system.
The return voltage drop could be reduced by
* Very short substation distances, so that the voltage drop is reduced.
* Reinforcing the return current path by extra conductors.
It also may be posible to have a section of track where only one train is permitted between DC substations, so there is no AC train on the section to suffer problems of the induced voltage. However cross bonding between adjacent lines would have to be removed or else an AC train on the other line could be affected.
It should be posible to modify most of our dual voltage stock for 4th rail operation. It is normal practice to keep traction supplies isolated from the equipment case and only connect to ground via axel brushes so that that current does not go through bearings. Just remove these brush connections and fit some 4th rail shoes.
My favored option would probably be to dual electrify the lines from Harrow on the hill to Amersham although I see dual voltage stock as a reasonable option
A DC train which draws several thosand amps creates a voltage drop along the running rails which can be in the order of 10's of volts and easily cause transformer problems. However with LU 4th rail system the return current goes along an isolated rail so there should not be a problem. The electrical equipment on LU trains is tested to ensure it is isolated from the coach body and the equipment could run with either rail earthed. Nominaly the electrical system is floating and this has the advantage that an earth fault on either live rail could be tolerated.
Where 3rd rail and OHL electrifications exist a number of measures would be required to reduce the problem of imposing a DC voltage on the AC system.
The return voltage drop could be reduced by
* Very short substation distances, so that the voltage drop is reduced.
* Reinforcing the return current path by extra conductors.
It also may be posible to have a section of track where only one train is permitted between DC substations, so there is no AC train on the section to suffer problems of the induced voltage. However cross bonding between adjacent lines would have to be removed or else an AC train on the other line could be affected.
It should be posible to modify most of our dual voltage stock for 4th rail operation. It is normal practice to keep traction supplies isolated from the equipment case and only connect to ground via axel brushes so that that current does not go through bearings. Just remove these brush connections and fit some 4th rail shoes.
My favored option would probably be to dual electrify the lines from Harrow on the hill to Amersham although I see dual voltage stock as a reasonable option
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