Besides shifting mountains of mail, it’s had an interesting life, from hiding priceless art and artifacts to housing a Hollywood film set.
Today just a handful of engineers keep the system in good condition, but in 2017 a looping mile of track will be opened to the public as a heritage site, part of a new museum, bringing the dark and empty complex back to life.
Carrying mail across Victorian London was slow because of the density of traffic, and from 1850 onwards the Post Office explored options for an underground tunnel to move letters between its eight London sorting offices. But it wasn’t until 1913 that work began on a major system.
By 1917, the Post Office had completed the tunnels and was ready to lay track and infrastructure, but the country’s resource were being focused on the World War One, and the scheme was halted.
Instead, from January 1918, the National Portrait Gallery co-ordinated the storage of all kinds of treasures in the complex, including the Rosetta Stone, to protect them from German attacks.
When the system finally began carrying mail in 1927, it was a world-first: there had been no previous electric railway with driverless trains. It was known as the Post Office Underground Railway, until its sixtieth anniversary in 1987, when its name was changed to Mail Rail.
At its peak Mail Rail carried four million letters every day, for 22 hours per day.
The tunnels and trains themselves changed little from the 1930s to the 1980s, sticking to their task as vital arteries in the nation’s mail system, tucked away out of view.
Above ground saw great upheavals, with only a few of them trickling down. During World War Two, Mail Rail was called into use as a dormitory for servicemen.
Later it became a film set, featuring in Hudson Hawk as the Vatican Underground Postal Railway. The film may not have been critically acclaimed, but the Post Office used the profits to throw a party for disadvantaged local children.
The staff also held incredible Christmas parties deep underground, including light shows and a Santa train.
Changing technology led to the closure of Mail Rail in 2003.
Since then it has been largely frozen in time, with old rotas still hanging on the walls but soon the general public will get access to this hidden part of London for the first time in its history.
Among the supporters of The Postal Museum are Royal Mail, Post Office and the Heritage Lottery Fund.