The Berlin U-Bahn (German: [ˈuː baːn]; short for Untergrundbahn, "underground railway") is a rapid transit system in Berlin, the capital and largest city of Germany, and a major part of the city's public transport system. Together with the S-Bahn, a network of suburban train lines, and a tram network that operates mostly in the eastern parts of the city, it serves as the main means of transport in the capital. The Berlin U-Bahn is the most extensive underground network in Germany.In 2006, travel on the U-Bahn was equivalent to 122.2 million km (76 million mi) of car journeys.