Mail Rail
The GPO's underground railway was opened in December 1927 to link major railway stations with what were then London's major sorting offices. It runs about 21m (70ft) below ground for about 10.5km (6.5 miles), from Paddington in the west to Whitechapel in the east.
This underground - now called Mail Rail - did not carry passengers but only mail to avoid London's busy congested streets. It was built so that the large volume of mail arriving at or leaving from Paddington and Liverpool Street stations could be dealt with quickly.
Mail Rail is currently suspended as a service although the maintenance of the tunnels remains.