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
Transport in London the National Rail system. There are several extensions planned for the system, such as expanding its coverage and further integrating it into the Underground system. London is the focal point of the British railway network, with 18 major stations providing a combination of suburban, intercity, airport and international services; 14 of these stations are termini and 4 are through stations, while two others are both. Most areas of the city not served by the Underground or DLR are served by suburban heavy rail services into one of these stations. These suburban rail services are not part of Transport for London