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
South London line South London line The South London line is an Inner London part of the London Overground rail network. The line is run together with the East London line to provide direct services between and . It consists of eight stations, one of which marks the crossover into the East London line network and runs . Most of the line is on high viaduct over other transport infrastructure. Interchanges with the London Underground are at and the closest on its London Overground extension is . The line is in Travelcard Zone 2. From the early 20th century until 2012 a shorter