For nearly a century, and unknown to many Londoners, a postal railway network operated 21 metres below the streets of the city, shuttling around the country’s correspondence. At its peak, Mail Rail ran for 6½ miles from Paddington to Whitechapel, carrying letters and facilitating communication across the capital.

Visitors will be able to take a 15-minute ride on the underground train

The service was the world’s first driverless electric railway when it opened in 1927 – an ancient DLR of sorts – introduced as a response to heavy traffic on the streets of London, which was delaying the delivery of post. The railway was closed in 2003 but a section of it will reopen on 28 July as part of the new Postal Museum & Mail Rail in Clerkenwell, central London.

In contrast to the letters and packages the former railway transported, the new version will carry visitors through the tunnels, providing them with a 15-minute underground ride. Through the ride’s audio-visual technology, visitors will experience the blitz and other tumultuous events that occurred during the railway’s working life.

The Mail Rail tunnels

The tunnels are a site of history and mystery: urban legend says that they were used to hide the Rosetta Stone during the first world war, and more recently they were discovered and illicitly explored by the Consolidation Crew, a group of “place hackers” committed to discovering and documenting the world’s restricted spaces. The reopening provides an opportunity for the public to learn about the city’s underground heritage first hand.

The above-ground museum will feature exhibitions, as well as displays showing how the system worked.

Above ground, in the same location as the former Mount Pleasant depot, the Postal Museum will feature an exhibition on Mail Rail. The exhibition explains its inception, maintenance and legacy, while also spotlighting the stories of individuals whose lives were connected to the system. The rest of the museum features educational play spaces for children and is home to a vast archive of mail-related content spanning 400 years, such as a confiscated version of James Joyce’s Ulysses and telegrams sent by passengers on the stricken Titanic.

• Admission (exhibitions and mail ride) adults £14.50, children (1-15) £7.25, postalmuseum.org. Open daily 10am-5pm (except 24-26 December)