The Northern line is a London Underground line, coloured black on the Tube map. The section between Stockwell and Borough opened in 1890, and is the oldest section of deep-level tube line on the network.
For most of its length it is a deep-level tube line. It is unique in having two different routes through central London – the Charing Cross or West End branch, serving the central part of zone 1, and the Bank or City 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, of which 36 have platforms below ground. wikipedia