It was a world first - and today the London Underground turned 150 with a series of events celebrating the milestone.
The first stretch of the world-famous network opened on January 9 1863, with the first passenger journeys taking place the following day.
Since that first stretch was opened between Paddington and Farringdon, then known as the Metropolitan Railway, the network has expanded to 12 lines.
Celebrations include additional heritage rail trips using steam trains, two new £2 coins and a set of 10 special stamps issued by Royal Mail.
London Mayor Boris Johnson described the network as "arguably the best and most iconic, underground transport system in the world."
It now carries 1,107 million passengers a year and serves 270 stations linking central London to Surrey, Kent, Essex, Hertfordshire, Middlesex and Buckinghamshire and provides jobs for around 19,000 people.