The Post Office Railway is a 2 ft narrow gauge, driverless underground railway in London that was built by the Post Office with assistance from the Underground Electric Railways Company of London, to transport mail between sorting offices. Inspired by the Chicago Tunnel Company, it opened in 1927 and operated for 76 years until it closed in 2003. A museum within the former railway was opened in September 2017.

The Underground Electric Railways Company of London Limited (UERL), known operationally as the Underground for much of its existence, was established in 1902. It was the holding company for the three deep-level "tube" underground railway lines opened in London during 1906 and 1907: the Baker Street and Waterloo Railway, the Charing Cross, Euston and Hampstead Railway and the Great Northern, Piccadilly and Brompton Railway. It was also the parent company from 1902 of the District Railway, which it electrified between 1903 and 1905. The UERL is a precursor of today's London Underground; its three tube lines form the central sections of today's Bakerloo, Northern and Piccadilly lines.