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–Aylesbury line London–Aylesbury line The London–Aylesbury Line is a railway line between London Marylebone and Aylesbury, going via the Chiltern Hills; it is operated by Chiltern Railways. Nearly half of the line is owned by London Underground, approximately – the total length of the passenger line is about with a freight continuation. The line is part of the former trunk route, the Great Central Main Line. The route towards Aylesbury opened in stages between 1868 and 1899: The Great Central Railway (GCR) decided to build a main line called the "London Extension" from Annesley Junction north of Nottingham to London via the