By the early 20th century, the advent of the motor car had led to increasing congestion on the streets of London. This in turn affected the speed of mail deliveries in the capital.
To counteract this, a plan was devised to construct an electric railway underground, entirely separate from the tube, which would transport mail between Paddington Station and The Eastern District Post Office in Whitechapel Road. The plan was submitted by the Postmaster General to the Cabinet in 1912 and the permission to go ahead was granted by the Post Office (London) Railway Act, 1913.
The railway was opened in late 1927 and ran for 76 years before being suspended in 2003.