Mail Rail was a miniature underground railway, opened by Royal Mail in 1927. It ran from the western sorting office in Paddington to its eastern counterpart in Whitechapel, with six initial stops. The most important stop was beneath the main Mount Pleasant Sorting Office, connecting this postal hub with the suburbs.
The Mail Rail trains that ran along this 6.5 mile line were driverless and handled pretty much ever single letter which passed through London during the period the underground railway was in use.
It was replaced by a road transport network in 2003, when Royal Mail mothballed it for cost and practical reasons. No further use has been found for the tunnels, so there is a small network of unused mini tube lines running beneath London on a east-west axis. Setting for a horror story anyone?