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
Attacks on the London Underground On 9 December, a van bomb partially detonated at the car park of Woodside Park station. No-one was injured but it caused evacuations and disruptions. A device exploded at an underground passageway at South Kensington station on 3 February 1993. On 20 December, a device exploded in a litter bin in Northfields station. On 21 December, coded bomb warnings from the IRA resulted in a paralysed London Underground system, as tens of thousands were evacuated from 100 tube stations during the morning rush hour. In 2005 two groups of Islamist extremists attacked a number of underground lines and bus routes