D7666 wrote:

I would have thought the most significant length constraint would be
Willesden Junction (for LO trains obviously not SN). To extend that to
8-car would involve bridging WCML and that would not come cheap. As
there would be no benefit to LO in 8car trains if Willesden Junycion
were never done, the entire cost of 8car works on WLL would be born by
the SN operation.

I expect WJ (HL) won't be anything like as difficult to extend to 8 car
length once the current 4 car extension is completed. Getting across the LL
tracks, which is underway now, is the main problem to solve. Having said
that - I'm not too sure where the new reversing siding is going with respect
to the new platform ends - that could prove a limiting factor in the
eastward direction as well...

A bit academic though unless Shepherds Bush and Imperial Wharf have room for
extension.

Paul S