The Northern line is a London Underground line, coloured black on the Tube map.
For most of its length it is a deep-level tube line. There were about 252,310,000 passenger journeys in 2011/12 on the Northern line, making it the second busiest line on the Underground. (It was the busiest from 2003 to 2010.) It is unique in having two different routes through central London – the Charing Cross branch, serving the central part of zone 1, and the Bank branch, serving the eastern part of that zone. Despite its name, it does not serve the northernmost stations on the network, though it does serve the southernmost station (Morden), as well as 16 of the system's 29 stations south of the River Thames. There are 50 stations on the line, 36 of them below ground.
The line has a complicated history, and the current complex arrangement of two northern branches, two central branches and the southern branch reflects its genesis as three separate railway companies, combined in the 1920s and 1930s. An extension in the 1920s used a route originally planned by a fourth company. Abandoned plans from the 1920s, to extend the line further southwards, and then northwards in the 1930s, would have incorporated parts of the routes of two further companies. From the 1930s to the 1970s, the tracks of a seventh company were also managed as a branch of the Northern line. An extension from Kennington to Battersea is now planned, which will give the line a second southern branch to go with its two northern and central branches, or may see the line split into two distinct lines with their own identities.