The line was originally named the Great Northern & City Railway, and was intended to allow mainline trains of the Great Northern Railway to run from Finsbury Park directly into the City of London at Moorgate. Accordingly, the tubes were made large enough to take a mainline train, unlike those of the other tube railways being built in London at the time. However, the Great Northern was lukewarm about the scheme, and the GN&CR had to settle for a northern terminus in tube underneath Finsbury Park station. The line opened in 1904.
The GN&CR was bought in 1913 by the Metropolitan Railway, which operated today's Metropolitan, Hammersmith & City and East London Lines. The Met made various plans to link it to some point on the Circle Line or to the Waterloo and City Line, but these never came to fruition.
After the Metropolitan was nationalised (along with the other underground lines) in 1933, the line was renamed the Northern City Line and became part of the Edgware-Morden Line (which became the Northern Line in 1937) for operational purposes. As part of London Underground's "New Works" programme, plans were made to connect the Northern City Line to the surface at Finsbury Park, and then take over suburban branches to Alexandra Palace, High Barnet and Edgware. The existing Highgate branch of the Edgware-Morden Line would be connected to this network north of Highgate. By the time the Second World War started, the Highgate link and electrification of the Barnet branch were well under way, but work on the Northern City link was prevented.
The "Northern Heights" plan was dropped after the war, and the Northern City Line remained isolated from the rest of the network. Services were withdrawn from Finsbury Park in 1964 to make room for the Victoria Line. In 1970 the line was renamed Northern Line (Highbury Branch). The following year, an agreement was made to transfer the line to British Rail and connect it (as was intended by its original promoters) to the main line at Finsbury Park. The last London Underground services ran in October 1975, and British Rail services commenced in August 1976. The track is now owned by Network Rail, and services to Stevenage and Welwyn Garden City are provided by WAGN. The name "Northern City Line" has been revived to refer to the underground part of the route.
The most serious railway accident on the London Underground system occurred at Moorgate tube station on February 28, 1975, when a Highbury Branch train ran through the terminus at speed and crashed into the dead end of the tunnel beyond. The cause of the accident, which killed 43 people, was never determined.
The original generating station for the GN&CR was closed when the Metropolitan Railway took over, and became the studio of Gainsborough Pictures. After lying derelict for many years, it became a temporary venue for the Almeida Theatre, and is now being redeveloped as apartments surrounding a revived film studio.
Stations