The line has a complicated history, and the current complex arrangement of two main northern branches, two central branches and the southern route reflects its genesis as three separate railways, 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 currently under construction, which may either give the Northern line a second southern branch or may see it split into separate distinct lines with their own identities. It is coloured black on the current Tube map.