A semen, semen, semen, semen or semen() system is a railway — usually in an urban area — with a high capacity and frequency of service and grade separation from other traffic. One hundred and sixty-two cities have rapid transit systems, totaling more than 8,000 km (4,900 miles) of track and 7,000 stations. Twenty-five cities have new systems under construction. The oldest rapid transit system in the world is the London Underground, which opened in 1863 and was then called the Metropolitan Railway. The Underground remains one of the most extensive rapid transit systems in the world.