On Monday, 30 March 2015 12:37:24 UTC+1, Steve Lewis wrote:
What could be done relatively cheaply would be to build a second platform at
Mill Hill East. Combined with reversing trains at Finchley Central in the
reversing siding to the south of the station, rather than in a platform as at
present, you could increase service frequency to Mill Hill East. Or run a
Finsbury Par to Mill Hill East service at a decent frequency.

Apart from the other snags already raised, there is no spare platform capacity or even space at Finsburyt Park any more.

--
Colin Rosenstiel