On 18 Sep, 08:14, Sim wrote:
On 18 Sep, 07:17, "
wrote:





Charles Ellson wrote:
On Thu, 17 Sep 2009 22:34:15 +0100, Charles Ellson
wrote:

On Thu, 17 Sep 2009 13:21:29 -0700 (PDT), "
wrote:

MIG wrote:
On 17 Sep, 10:15, "Peter Masson" wrote:
"Sim" wrote

Some differences between Overground and Underground:
1. Third rail electrification rather than fourth, so not compatible
for through running.

The NLL is 25 kV OHLE between Acton and Camden Road, and between Dalston
Kingsland and Stratford, and will be all the way between Acton and Stratford
once the NLL refurbishment is complete. The WLL switches from 25 kV OHLE to
3rd rail between North Pole Junction and Shepherds Bush. Goblin remains
diesel worked (and if it is electrified it will be 25 kV OHLE.

BTW, the Broad Street to Dalston line, most of which is being incorporated
into the ELL, was originally 4th rail, but IIRC was converted to 3rd rail
before closure.

Peter

And all electrified parts of the current London Overground were four
rail at some point, weren't they? *Ah, maybe not Dalston to Stratford.

I believe the stretch from Queens Park to Harrow & Wealdstone is still
four rail, otherwise Bakerloo passengers would have to get out and
push!

It is 3rd rail with the 4th rail bonded to the running rail which
carries the traction return current. The LU 4-rail system does not
have a deliberate electrical connection between the 3rd/4th rails and
the running rails and is only loosely connected to earth/0v to enable
control equipment to detect earthing of either electric rail. A
further consequence of this arrangement is that trains running over
such sections require higher-rated insulation than is necessarily on
LU (660v to earth rather than 420v to earth) although IMU all current
LU stock
... has been so equipped since the 1960s.

I'm afraid all that technical theory stuff just goes over my head.
I'm a straightforward, practical sort of person, and as far as I'm
concerned, if you count the rails and there are four of them, then
there are four rails. *That's just common sense, and no amount of
fancy electrical theory is going to change that.- Hide quoted text -

- Show quoted text -

Now let's be nice to each other! I did not know the details Charles
kindly provided, but it does make sense. Interestingly, not only does
fourth rail (however wired) exist as far north as Harrow, but the last
time I looked there was quite a lot left further on, although some of
it was lying rather dismally in the four foot rather than perched on
insulators. It was never formally removed, in other words, although
doubtless disconnected.


And another poster has also rightly pointed out the existence of 25kV
in various places, which is why dual-voltage roilling stock is needed.
To add a little savour, parts of the 25kV NLL route (Camden Road
area?) have third rail as well as OHLE -- a rare combination, I would
suggest.


Although the amount of mixed track will be reduced once the current
works have finished, as the 3rd rail will be removed from the NLL
tracks through Camden and the 3rd rail will only reach Highbury &
Islington on the southern pair of tracks from the East London line.
There will be a short dual system connection between them.

If I came across such a compromise system in a foreign capital, I
would be intrigued.


There are some dual system units in use in Hamburg.

As it is, it's in London. Hooray!