What is the Northern line?

The Northern line is a deep-level tube line on the London Underground, coloured black on the Tube map.
It carries more passengers per year than any other Underground line.

Despite its name, the Northern line does not serve the northernmost stations on the Underground network although, ironically, it does serve the southernmost station and serves 16 of the Underground system’s 29 stations south of the River Thames: a proportion higher than any other line. Of the 50 stations on the Northern line, 36 are underground.