The core of the Northern line evolved from two railway companies – the City & South London Railway (C&SLR) and the Charing Cross, Euston & Hampstead Railway (CCE&HR). In conjunction with the works to integrate the two lines, two major extensions were undertaken; northwards to Edgware in Middlesex and southwards to Morden in Surrey.
The Edgware extension utilised unused plans dating back to 1901 for the Edgware and Hampstead Railway (E&HR) which the UERL had taken over in 1912. It extended the CCE&HR line from its terminus at Golders Green to Edgware in two stages – to Hendon Central in 1923 and to Edgware in 1924. The line crossed undeveloped open countryside and, apart from a short tunnel north of Hendon Central, was on the surface.