The Northern line is an underground railway in London, that is coloured black on the London Underground Tube map. The line's two branches carry 206.734 million passengers per year—the highest on the London Underground system. For most of its length it is built as a deep-level tube line. Despite its name, it is the Underground line that extends furthest south. There are 50 stations on the Northern line, of which 36 are underground.

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 that were brought together and combined in the 1920s and 1930s. The original routes were extended several times so that by 1926 the line served Edgware in the north and Morden in the south. Ambitious plans to take over and incorporate London & North Eastern Railway's Northern Heights branch lines and extend the line to Bushey were mostly cancelled following the Second World War. (Full article...)