 The line is one of just two Underground lines to cross the Greater London boundary (the other being the Central line).It is the only Underground line with an express service at peak times; the resulting longer distance between stations means trains can achieve the system's highest speeds of over on some sections.In 1863 the Metropolitan Railway began the world's first underground railway between Paddington and with wooden carriages and steam locomotives, but its most important route became the line north into the Middlesex countryside, where it stimulated the development of new suburbs.