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
Greater London boundary with Kent, at 245 metres (804 ft). Central government has implemented small boundary changes. The greatest were the 1969 transfers of Farleigh to Surrey and Knockholt to Kent. Others have included exchange of two Thames islands with Surrey and adjustments during the 1990s to parts of the boundaries of three boroughs near the M25. The only part of Greater London outside the motorway is North Ockendon, the furthest land unit from its centre. The majority of Greater London forms the London low emission zone. The London postal district does not cover all of Greater London. The Mayor of London