Rapid transit system, also known as a metro, is a type of transportation system designed for high-capacity and high-speed travel in urban areas. Some synonyms for the term include subway, underground, tube, metro rail, and elevated rail. Subway is a term mainly used in North America and refers to an underground rail system. The underground is also a term used primarily in the United Kingdom for a similar transportation system, also known as the London Underground. Tube, another term mainly used in the UK, is derived from the round, tube-shaped tunnels through which the trains travel. Metro rail and elevated rail refer to above-ground rail systems used in some urban areas around the world.