From the longer Wikipedia page [1]
The Parkland Walk is a 4.5-mile (7.2 km) linear green walkway, in the London Boroughs of Haringey and Islington, which follows the course of the railway line which used to run between Finsbury Park through Stroud Green, Crouch End, Highgate and Muswell Hill to Alexandra Palace.
This path was once the route of part of the London and North Eastern Railway's (LNER's) line from Finsbury Park to Edgware constructed in 1867 by the Edgware, Highgate and London Railway, with the branch to Muswell Hill and Alexandra Palace added in 1874. Plans were published by London Underground in the 1930s for its incorporation as part of the Northern Line (The Northern Heights Plan) but the onset of World War II stopped the work at an advanced stage.
After the war the development plan was abandoned but passenger trains continued to run on this line until 1954. The service was reduced to freight haulage and tube traffic, until its final closure in 1970.
Tracks and infrastructure were removed and most of the platforms and station buildings demolished. The Parkland Walk was officially opened in 1984 following extensive re-surfacing and improvements to access.
The Friends of the Parkland Walk website is [2]