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
Romford–Upminster line am and 8 pm on Sundays. The journey time from one terminus to the other is approximately nine minutes. During the construction of the railway in 1892, the geologist T. V. Holmes discovered a five-metre layer of boulder clay underlying the gravel and sand in a section just north of St. Andrew's Park. This is now the Hornchurch Cutting Site of Special Scientific Interest, which exhibits the southernmost limit of the Anglian ice sheet 450,000 years ago, the furthest south that any ice sheet reached. Romford–Upminster line The Romford–Upminster line is a railway branch line in Greater London that connects