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 infrastructure although in some places, for example the Central line east of St Paul's station, tunnels are above each other. The running tunnels are on the right on the Victoria line between Warren Street and King's Cross St. Pancras, allowing cross-platform interchange with the Northern line between northbound and southbound trains at Euston. Six of the 32 London boroughs are not served by the Underground. All of these are south of the River Thames: Bexley, Bromley, Croydon, Kingston, Lewisham and Sutton. The borough of Lewisham used to be served by the Underground on the East London line – now part of