being just outside the HS2 blight zone ?
On the subject of the Northern Line I always found the most interesting thing to be the ghost station , North End Station / Hampstead Garden Suburb , between Golders Green and Hampstead. You could see the platform in the gloom as you passed it.

This is a ghost station on the Northern Line which would have served a new residential development being planned for the north of the Heath on part of Eton Colleges Wyldes Farm. However, although larger-diameter station tunnels and low-level passageways were excavated as part of the works to extend the line from Hampstead to Golders Green which began in 1903, Hampstead Heath Extension council was also formed in 1903 by Henrietta Barnett and the purchase of the 80 acres of development land, which came to be known as the Heath Extension, was completed in 1907. Work on the station was ended in 1906 after it became apparent that the removal of the proposed residential development would significantly reduce the number of passengers using the station. The uncompleted platforms and lower passageways remain, bricked-off from the tracks. The remaining 243 acres of Wyldes farm were transferred to a trust in 1907 to become the site of Hampstead Garden Suburb, Henrietta Barnets utopian experiment in suburban planning.
During the 1950s the station, which at 200 ft below ground level would have the deepest in the network, was rumoured to be London Transport's potential emergency headquarters in the event of a nuclear detonation in or near London and at around this time access to the station from the surface in the form of a rectangular staircase was finally provided. Later, it is believed to have become one of the control centres for the Underground's floodgate system. Signs on the gate and on the building's door indicate that the site is now a designated emergency exit point for the Underground network.
http://www.hampsteadramblers.org.uk/self-guided-walks/15-themed-walks/7-ghosts-hampstead-and-highgate-walk.html
Although Edgware is the end of the line there are 3 curious humps in open land to the north of Edgware. Story has it they considered extending the line north to Elstree / Boreham Wood.
Early sixties when I was considerably fitter and the old Northern Line Strand Station was still there I could get in and out of town for the jazz clubs from Edgware for 6 pence in old money. The ticket collector was in the lift which I could outrun up the long winding staircase which most in their right mind wouldn't have even consider using at a leisurely pace.
The Northern line Strand station was closed on 4 June 1973 to enable the construction of the new Jubilee line platforms. These platforms were constructed between the Bakerloo line and Northern line platforms together with the long-missing below-ground interchange between those two lines. In anticipation of the new interchange station, from 4 August 1974 Charing Cross was renamed Charing Cross Embankment. The Jubilee line platforms and the refurbished Northern Line platforms opened on 1 May 1979 from which date the combined station including Trafalgar Square was given its current name; simultaneously Charing Cross Embankment reverted to the original BS&WR name of Embankment, ending 109 years of association with the name Charing Cross. http://en.wikipedia.org/wiki/Charing_Cross_tube_station