Piccadilly line extension to Cockfosters

The Piccadilly line extension to Cockfosters added eight new stations to the northern end of London Underground's Piccadilly line. The extension through north London from Finsbury Park to Cockfosters was opened in three stages between 19 September 1932 and 31 July 1933.

When the Great Northern, Piccadilly and Brompton Railway (GNP&BR, precursor of the Piccadilly line) opened in December 1906, its northern terminus was at Finsbury Park where it had an interchange with the Great Northern Railway (GNR) and the Great Northern & City Railway (GN&CR). To obtain approval for the railway's construction, the GNP&BR had, like the GN&CR before it, had to accept a GNR veto over further extensions north in competition of latter's suburban passenger services from King's Cross.

Very soon after the GNP&BR opened it was clear that the termination of the line in urban Finsbury Park rather than further out of central London in more suburban Wood Green, Southgate or Tottenham had been a mistake. Passengers leaving the GNP&BR and the GN&CR at Finsbury Park preferred to transfer on to trams and buses for the continuation of their journeys, rather than use the GNR as it had hoped. This caused much inconvenience and congestion in and around the station at Finsbury Park.

Calls for a solution to the congestion at Finsbury Park were frequent almost from the GNP&BR's opening and after the First World War the campaign for extending the line gathered strength. In 1921, the Municipal Borough of Tottenham, sent a resolution to the Government, suggesting the desirability of extending one of the two lines then terminating at Finsbury Park to Seven Sisters Corner. In the face of mounting pressure the London and North Eastern Railway (LNER) continued the opposition of its predecessor, the GNR, to a northward extension of the Piccadilly line although it did begin developing plans for the electrification of its own suburban services.

...
Wikipedia