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 Standard Stock and were later reclassified to become Classes 485 and 486. Apart from those cut up following accidents at Ryde depot, and other early withdrawals, most of these units were withdrawn between 1988 and 1991, when they were replaced by London Underground 1938 Stock. In October 1990, five cars were returned to London Transport, to form a Heritage Train. They worked under their own power between Fratton and Wimbledon, and were displayed at an open day at Morden Depot, celebrating 100 years of the Northern line. Standard Stock is used to describe six experimental cars built by five manufacturers in 1923,