--- Alistair Bell said:

I'd be more interested in the feasibility of turning the Northern City
into a DLR line to East Finchley... the tunnelling would be much
shorter (but again with vault issues)

And also the issue of getting the DLR from directly below the Northern
line at Bank to directly above it at Moorgate. Apart from that, it's a
great idea, Alistair. It would relieve overcowding of the Northern
line's City and Edgware And it would give people in Essex Rd and
Drayton Pk a proper weekend/evening service for the first time in ages!

Divert existing Moorgate services over TL3K to South London (hey, the
313s are ideal for that job)

Aren't there loading guage problems on the Thameslink tunnel? IIRC only
319s are ideal, anything else is adequate at best. But maybe enlarging
the tunnel is part of TL3K. It's been so long since it was first
proposed, I can't even remember!

It would be nice to run the DLR all the way on to Mill Hill (or even
via Mill Hill to Edgware!), but track-sharing and/or grade separation
issues might stymie that.

It'd need a flyover to take the DLR over the northbound Northern track
(Still cheaper than all that tunnelling between Moorgate and Bank!) and
an extra platform at Finchley Central if there's space. Otherwise,
single-track it through FC, add a passing loop between there and Mill
Hill East, and rely on the computers and moving blocks to schedule the
trains round it efficiently. (cf. Pudding Mill Lane)