Mill Hill East tube station in the context of "Single-track railway"

The Northern line is a London Underground line that runs between North London and South London. It is printed in black on the Tube map. It carries more passengers per year than any other Underground line – around 340 million in 2019 – making it the busiest tube line in London. The Northern line is unique on the network in having two routes through Central London, two northern branches and two southern branches. Despite its name, it does not serve the northernmost stations on the Underground, though it does serve the southernmost station at Morden, the terminus of one of the two southern branches.

The line's northern termini, all in the London Borough of Barnet, are at Edgware and High Barnet; Mill Hill East is the terminus of a single-station branch line off the High Barnet branch. The two main northern branches run south to join at Camden Town where two routes, one via Charing Cross in the West End and the other via Bank in the City, continue and then join at Kennington in Southwark. At Kennington the line again divides into two branches, one to each of the southern termini – at Morden, in the borough of Merton, and at Battersea Power Station in Wandsworth.