The history of rapid transit began in London with the opening of the Metropolitan Railway, which is now part of the London Underground, in 1863. By World War I, electric underground railways were being used in Athens, Berlin, Boston, Buenos Aires, Budapest, Glasgow, Hamburg, Liverpool, New York City, Paris, and Philadelphia. In the 21st century, China became the world's leader by number of rapid transit systems as well as the fastest growth of such systems, and many other Asian countries began construction of their own rapid transit systems.