The London Northern 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 that were brought together and combined in the 1920s and 1930s.

An extension in the 1920s used a route originally planned by a fourth company and abandoned plans to extend the line southwards in the 1920s and northwards in the 1930s would have incorporated parts of the routes of a two further companies. Each platform is 110 metres long