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
Central line (London Underground) Total track length is , of which is in tunnel; this track is electrified with a four-rail DC system: a central conductor rail is energised at −210 V and a rail outside the running rail at +420 V, giving a potential difference of 630 V. The single-track line north of Epping, which closed in 1994, is now the Epping Ongar heritage railway. shuttle services operate on weekends between North Weald and Ongar and North Weald and Coopersale. These do not call at Blake Hall, as the station platform was removed by London Transport after the station closed, and the remaining