The Post Office Railway, also known as Mail Rail, is a 2 ft (610 mm) 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 move mail between sorting offices.
The line ran from Paddington Head District Sorting Office in the west to the Eastern Head District Sorting Office at Whitechapel in the east, a distance of 6.5 miles (10.5 km). It had eight stations, the largest of which was underneath Mount Pleasant, but by 2003 only three stations remained in use because the sorting offices above the other stations had been relocated.
In 1911 a plan evolved to build an underground railway 6 1⁄2 miles (10.5 km) long from Paddington to Whitechapel serving the main sorting offices along the route; road traffic congestion was causing unacceptable delays. The contract to build the tunnels was won by John Mowlem and Co.Construction of the tunnels started in February 1915 from a series of shafts. Most of the line was constructed using the Greathead shield system, with limited amounts of hand-mining for connecting tunnels at stations.(ref:Wiki)
Royal Mail eventually closed the system in 2003, stating that operational costs were five times higher than using road transport for the same task—which is beautifully ironic, as the London Post Office Railway was originally built because of massive road traffic congestion in the early 1900s.
From September 2017, the Mail Rail, as it later became known, will ride again—but it’ll carry people rather than post.
you can book a ticket on https://www.postalmuseum.org