London Underground infrastructure Tube Programme, investigating replacing the trains for the Bakerloo and Piccadilly lines, is looking for trains with better energy conservation and regenerative braking. The Underground serves 270 stations. Fourteen Underground stations are outside Greater London, of which five (Amersham, Chalfont & Latimer, Chesham, and Chorleywood on the Metropolitan line and Epping on the Central line) are beyond the M25 London orbital motorway. The longest distance between two stations is between Chalfont & Latimer and Chesham on the Metropolitan line. The shortest distance between adjacent stations is the between Leicester Square and Covent Garden on the Piccadilly line. The station furthest
London Underground S Stock (ex-Metropolitan Railway) used with two 1925-built prototype driving motors. In 1925, two driving motor cars were built as prototypes for what would later become the T stock. In the London Passenger Transport Board numbering scheme, they became 2598 and 2599. After the trailer cars were withdrawn from service in the 1940s, the two driving motor cars remained in service on the East London Line, being used with a 1923-built control trailer. These three cars were withdrawn in 1950. London Underground S Stock (ex-Metropolitan Railway) S stock is the designation given to one train of eight cars by London Underground in the 1930s.