Mail Rail train driver Penelope Veck drives through the underground eastbound Mount Pleasant Sorting Office station on July 28, 2017 in London, England. [Photo/VCG]

LONDON - A miniature railway that once whisked more than 4 million letters and parcels a day deep beneath London is to be brought back into use, as a heritage tourist attraction.

From the 1920s the network of rail tunnels was operational day and night along the little known "Post Office" railway, linking major postal sorting offices in the capital.

Tickets have now gone on sale for the first journeys along the route when it officially opens to visitors on Sept 4.

The rail journey, in a specially built miniature rail carriages, is expected to become the star attraction at London's new Postal Museum which opened Friday to tell the story of Britain's Royal Mail.

The museum, opened by the Postal Heritage Trust, brings five centuries of British communications history to life, with displays and objects, such as a hand pistol issued to postmen in the past to protect them from robbers known as highwaymen.

The trust is now responsible for the massive collection of memorabilia, much of it on public display at the museum in Phoenix Place, Farringdon in central London.

Tickets are already selling fast for the railway experience, travelling on the 11-kilometer long "secret" rail network, that criss-crossed London's more famous Underground Rail network. The subterranean train network stretched from Whitechapel to Paddington, employing a staff of more than 200 postal workers. It enabled mail to be moved around London away from the crowded and busy streets above,.

The Postal Railway finally closed in 2003, but has remained mothballed until now.

One of the first people to be given a sneak preview of the new railway trains was Queen Elizabeth's daughter, the Princess Royal.

Because of the narrow width and height of the tunnels, two new trains have been adapted from the original design to enable them to carry passengers in miniature wagons.

A spokesman at the museum said: "For more than 75 years, the postal rail was a vital artery in Britain's communication network, hidden from view. Now our visitors will be able to experience it from Sept 4.