On Friday, 2 October 2015 21:55:43 UTC+2, Clive D. W. Feather wrote:
In message , David C
wrote:
I'm not saying it can't be done but it is a lot more complicated to signal
so tends to be avoided. The LUL 3rd & 4th rail electrified tracks lie
between platform 1 and the overhead electrified tracks, don't they? So the
Barking Riverside route won't go near platform 1 so why keep it in service?

Sorry but you are not making any sense.

As has been mentioned, 25 kV AC electrification has co-existed with
the under-ground since 1962without any problems.

I think there's a misunderstanding here.

There are serious problems with having third (or third/fourth) rail on
the same line as 25 kV, to do with earthing and track circuits.
Therefore there's good reasons to avoid that.

But, on the other hand, I don't believe there's any significant reason
not to have 25 kV and third rail on adjacent tracks. Particular with
platforms in between.

When this topic has been discussed in some detail in the past, the issue with dual electrification of 25 kV overhead and 3rd rail is related to the mutually exclusive requirements for how the running lines are bonded to earth for the return current. My understanding of the signalling related complications (not impossible to overcome if for example axle counters are used) relate to the need to cope with both DC and 50 Hz AC return current in the tracks (and related harmonics) in company with track circuits. All of these concerns, however, seem to relate to the presence of the track return current. In the case of LU style 4th rail, the DC return current is not via the running lines, the 4th rail is electrically isolated. What, therefore, is the issue with 25 kV AC and LU 4th rail combined electrification?

Robin