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
Safety on the London Underground train. Several stations on the Jubilee line extension are fitted with platform edge doors, which prevent people from falling or jumping onto the tracks. Terrorism in the London Underground has been a major concern because the Underground's importance makes it a prime target for attacks. Many warnings and several attacks, some successful, have been made on the Underground, the most recent on 21 July 2005, although in that case only the detonators exploded. The most recent attack causing damage was on 15 September 2017, when a man detonated a homemade bomb on a District line train at Parsons Green station.