London Underground at +420 V, giving a potential difference of 630 V. On the sections of line shared with mainline trains, such as the District line from East Putney to Wimbledon and Gunnersbury to Richmond, and the Bakerloo line north of Queen's Park, the centre rail is bonded to the running rails. The average speed on the Underground is 20.5 mph. Outside the tunnels of central London, many lines' trains tend to travel at over 40 mph in the suburban and countryside areas. The Metropolitan line can reach speeds of 62 mph. The London Underground was used by 1.379 billion passengers in
Attacks on the London Underground in London. On 7 July 2005, bombs exploded on Underground trains between Aldgate and Liverpool Street stations, Russell Square and King's Cross St. Pancras stations and Edgware Road and Paddington stations. A double-decker bus at Tavistock Square was also destroyed. The bombs were detonated by four homegrown terrorist suicide bombers. The explosions killed 52 people and resulted in over 700 injuries. Four more attacks, unconnected with those on 7 July, were attempted on 21 July 2005 at Shepherd's Bush Market, Warren Street and Oval stations, as well as on a bus in Bethnal Green. In these incidents, each bomb detonator