 Robert Selbie had fought vigorously for the independence of the Metropolitan Railway , and had successfully preserved the MR 's independence during the grouping of 1923 , which had merged almost all of Britain 's railways into four companies . However , on 17 May 1930 he died suddenly , and his successors acceded to pressure from the Ministry of Transport to merge with London 's other underground railways . On 1 July 1933 the merger brought all of London 's underground railways aside from the short Waterloo & City Railway , under public ownership as part of the newly formed London Passenger Transport Board ( LPTB ) . The Metropolitan Railway became the Metropolitan line of the London Underground . 
