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
London Underground D78 Stock designated as the Olympic 2012 Train with London Underground Chief Operating Officer Howard Collins carrying the Olympic Torch from Wimbledon to Wimbledon Park. This is the only London Underground train to be an Olympic Torch train. De-icing equipment was fitted to trailers 17000 to 17048 (evens). The mid-life refurbishment was the first to be carried out under the Public–private partnership (PPP), by Metronet, and was delayed until contract negotiations were completed. A prototype unit of three cars (7008/17008/8008) was prepared by London Underground's Train Modification Unit at Acton Depot in 2001. This had some detail differences from the eventual refurbishment,