The Central Line
The Central Line runs east to west across London and is represented on the London Tube Map by a red line. 20 of the line's 49 stations are below ground and at 46 miles it is the longest and second busiest line in the London Underground.
Initially operated by electric locomotives, the line was opened in June 1900 after years of delays and extensions. The Central London Railway was incorporated in1891 but the completion date was moved back twice, in 1894 and 1899.
The tunnels that form the underground part of the Central Line were originally 11 feet 8¼ inches in diameter which was increased on the curved sections but trimmed to 11 feet 6 inches closer to the stations. The 45 ton locomotives caused problems with vibration.
The tunnels were re-aligned and expanded in the 1930s and at the same time that the stations themselves were lengthened, and the line was converted to the standard four-rail electrical layout in 1940, but the tubes themselves were no longer round (because of how they had been enlarged) leading to some minor differences with the electrical rails.
These variations meant that the Central Line stock could not run on any other line, and the oddities can still be seen today - the insulating pots on the positive rail being placed on small concrete mounds to provide the extra height needed.