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 is proposed that the Northern line be extended to Nine Elms and Battersea. Construction started in 2015, and it is hoped the stations will open in 2020. There are suggestions that the Bakerloo line be extended to Lewisham, and then taking over services on the Hayes Line to relieve capacity on the suburban rail network. The London Borough of Hillingdon has proposed that the Central line be extended from West Ruislip to Uxbridge via Ickenham, claiming the extension would cut traffic on the A40 in the area. The signalling system on the Northern line is being replaced to increase capacity