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
TfL Rail and in Essex. That branch comprises the first 14 stations on the Great Eastern Main Line, with interchange at Shenfield for medium- and long-distance services beyond to East Anglia. In May 2018 TfL Rail also took over operation of the Heathrow Connect service between in central London and Heathrow Airport to the west (Heathrow Terminal 4 station being the terminus). That route has nine stations. The service is operated by MTR Corporation under contract to Transport for London (TfL). Between May 2016 and May 2017, TfL Rail carried over 47 million passengers on the Shenfield branch. In June 2013 TfL