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
History of the London Underground 2008. On 7 July 2005 three bombs exploded on underground trains, two on the Circle line at Aldgate and Edgware Road and the third on the Piccadilly line between King's Cross and Russell Square. Later the same day a bomb exploded on a bus in Woburn Place. Four suicide bombers had killed themselves and fifty-two other people, and it took over a month before underground services had been restored. In 2007 the East London line, operated as an isolated shuttle since 1939, closed so that it could be converted into a London Overground line. In December 2009 the Circle line