The London Underground's Northern Line has 50 stations and is represented on the London Tube map by a black line and is the busiest line on the Tube, handling almost 207 million journeys every year. There are two branches through London which contribute to this. 36 of the stations on the Northern Line are underground.
Reflected in its complicated map, the Northern Line has a complicated history and branch arrangement. It originally began as three distinct and wholly separate lines with their associated companies which were being combined into one during the 1920s-30s. There were also plans for other branches and extensions which would have complicated matters further by introducing more companies. In all, the Northern Line is made up from the lines of seven different companies.
Image from the Northern Line article on Wikipedia.
As can be seen, there are several branches:
There are also four closed stations: King William Street, City Road, North End and South Kentish Town.