The ground under London is criss-crossed by countless tunnels and subterranean transport links, one which was little known about by the general public was the underground railway run by the Post Office between Paddington station and Whitechapel. The system ran from 1927 until 2003 with the network's largest station at Mount Pleasant (where my great uncle worked after the War incidentally so he may have come into contact with the system). The driverless electric railway ran efficiently and barely noticed for many years and could carry mail from Paddington to the East End of London in just 26 minutes.
A new museum at Mount Pleasant is going to be opened soon and by 2020 a loop of Mail Rail will be reopened and this time carrying visitors not parcels! I am so going to go on this.