The London Underground is a form of public transport in London. It is a rapid transit system that uses electric trains. It is the oldest underground railway in the world. It started running in 1863 as the "Metropolitan Railway". After the opening the system was copied in many other cities, for example New York and Madrid. Even though it is called "the Underground" about half of it is above the ground. The "Tube" is a slang name for the London Underground, because the tunnels for some of the lines are round tubes running through the ground. The Underground serves 270 stations and over 408 km of track. From 2006 to 2007 over 1 billion passengers used the underground.