In article , Nick Cooper
writes
1) FP-AP/HB "only" needed electrification, as the track and stations
were all there (not too up on signalling, but IIRC some work was done
in this area) and, indeed, in use.

Highgate High Level was completely rebuilt, from outside platforms to a
central island.

And you still have the problem of how to reverse trains at Finsbury
Park. Doing it in the existing platforms just wouldn't work - there was
far too much surface traffic to handle (say) 6 to 10 tph reversing. So
you need the new flyover and the new platforms. Once that's done,
upgrading the slope to Drayton Park is a no-brainer.

3) The physical linkage of the AP branch and the NCL, with a
relatively short length of tunnel, where ever it's placed.

Why? Drayton Park is on the surface.

although the advent of the Victoria line - if at all - would have
raised questions in the case of a tube-level linkage, rather than a
surface one.

Now there's a question: what would the Victoria Line have done if the
Northern Line had been extended? Actually, the answer is simple: do what
was actually done with the tube platforms, including abandoning the
MOG-FP shuttles. Cockfosters to Moorgate passengers have a choice of two
cross-platform changes or one staircased one.

In terms on in-car
diagrams I would imagine one similar to the "with NCL" one of the
early-1970s, showing both distinct sections under a common management.

That sounds quite a good approach, actually.

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