Traction current

[q8]

Can someone answer a query?

I know the original reason for it's introduction but is it really needed for todays underground to have the 4th (negative) rail? Surely third rail only would suffice nowadays given the insulating properties of modern materials. The return path via the running rails would not affect AC signalling circuits as impedance bonds (or modern equivalent) would be fitted. The Glasgow sytem. the Moscow Metro (also in "tube" type tunnels) and others around the world manage perfectly well with third rail only so why not LUL? Think of the money that could be saved in steel/track maintainance/ ancilliary equipment and infrastruture. Is it viable?

While they are about it they could adopt the bottom contact conductor rails as used on the Berlin S Bahn

[Tom]

The problem is that Impedance bonds are large and expensive. The most simple track circuit around for 3rd rail is a lot more complicated and I'm sure there are other reasons which I can't think of now.

Saying that, it would make walking along the track (especially down the tunnels) so much easier, and I would welcome it for that reason.

[Colin]

Third rail is better? Have you not noticed the signal problems on the wimledon and Richmond roads over the last couple of weeks?!

Seriously though, you'd be looking at loads and loads of money just to convert all the supplys, rails, trains, signal systems, etc. Plus the neccessary shutdowns while they convert it all, it wouldn't be worth the hassle.

[igelkotten]

Tom said:

The problem is that Impedance bonds are large and expensive. .

Large I can agree with, but are impedance bonds really that expensive? Two big steel pots filled with oil, four cables and two copper braids doesn't sound all that expensive to me, at least. Or have I missed something?

[q8]

In reply to Mr Colin's post I think a lot of the problems on the Wimbo and Richmond road is down the fact that the underlying bed of the track is ash even where you have ballast laid on top of it. That ash has been there for over a hundred years and is well waterlogged now. In my day you could often see pools of water in the cess and six foot. You could also see a lot of "track pump" with little fountains of water spurting up as trains passed over. This was especially noticeable with the old heavy steel BR and our own stocks. The worst places for this were Wimbledon Park and between Kew and Richmond. They probably know this but to replace the ash would mean removing all the ballast and re-profiling/drying/consolidating the ash and they don't want to spend the money.

[Colin]

I am aware of the actual cause of the signal problems - it just seemed like a good opourtunity to pick on Network Rail who actually maintain those sections! ;D

I believe the main problem with the wimbledon road is that LUL own the infrastructure, but, Network Rail maintains it. As there are very few (empty) mainline trains using it, there's not much of a financial incentive for Network Rail to sort out the root cause. Happy days!

[Deleted]

there is they still get the fines for it well they do have failures down there
like last friday when the richmond branch was shut for most of the day it costed NR £200,000

[q8]

like last friday when the richmond branch was shut for most of the day it costed NR £200,000.
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Gawd almighty, if it's costing NR that much a pop they ought to spend a couple million and remedy the situation A.S.A.P. as it's gonna cost them a damn sight more that £2M in the long run. They don't think longer than a week do they?

[Dmitri]

COLIN said:

you'd be looking at loads and loads of money just to convert all the supplys, rails, trains, signal systems, etc. Plus the neccessary shutdowns while they convert it all, it wouldn't be worth the hassle.

Surely, and I think this is the main reason for conversion to never take place (after all, there are much better uses of excess money - if there are any). Having said that, if LU decides to convert to the 3rd rail, I'd suggest bottom contact design (same as we use) as it doesn't have problems with show (and spring-loaded collector shoes greatly reduce risk of gapping).