In message , Mr Thant
writes

AIUI the Northern Heights tracks would have both continued parallel
along the east side of Finsbury Park station on a new viaduct (which
was partially built, since demolished). I don't think it involved a
direct connection to the ECML, though I'm by no means an expert on the
subject.

Ah yes, I think you're right - there was an indirect connection via the
Finsbury Park goods yard, but not a direct connection.

--
Paul Terry