Which London underground line opened first?

The London Underground (also known simply as the Underground, or by its nickname the Tube) is a public rapid transit system serving Greater London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom.

The Metropolitan line is a London Underground line that runs from Aldgate, in the City of London, to Amersham and Chesham in Buckinghamshire, with branches to Watford in Hertfordshire, and Uxbridge in the London Borough of Hillingdon. Coloured magenta (Pantone 235) on the tube map, the line is 41.4 miles (66.7 km) in length and serves 34 stations. In 1863 the Metropolitan Railway began the world's first underground railway between Paddington and Farringdon Street with wooden carriages and steam locomotives, but its most important route became the line north into the Middlesex countryside, where it stimulated the development of new suburbs.

The network has expanded to 11 lines, and in 2015–16 carried 1.34 billion passengers, making it the world's 11th busiest metro system. The 11 lines collectively handle approximately 4.8 million passengers a day.

The Metropolitan line was opened in 1863.

The District line was opened in 1868.

The Hammersmith & City line was opened in 1864.

The Northern line was opened in 1890.