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Londoners will be able to take a trip on the Post Office’s underground rail lines for the first time ever when the new exhibition opens this summer.

The 6.5 mile ‘Mail Rail’ network went out of use over a decade ago and has lain dormant ever since.

But with the launch of the new Postal Museum on July 28 at the Mount Pleasant sorting office in Clerkenwell, part of the Mail Rail is also being reopened.

Posters have sprung up on Tube trains and in stations inviting curious Londoners and tourists to book tickets for the trips, which officially launch on September 4.

Here’s everything you need to know about the Mail Rail.

When was the Mail Rail built?

The network of narrow tunnels was established in 1927, stretching for 6.5 miles from Whitechapel in the east to Paddington in the west.

The tunnels, 70 ft below street level, criss-crossed tube lines and linked six sorting offices with the mainline stations at Liverpool Street and Paddington.

How did it work?

At its peak, more than four million letters were carried on the underground network each day. The service operated for 22 hours a day, employing over 220 staff.

The tunnels are much narrower than the Tube equivalents, with the main line measuring just 9 ft across. Two tracks run down it, although they split into two separate – even narrower – tunnels just before stations.

The stations are not as deep as the tunnel, reducing the distance to lift mail to and from the surface. It also means the trains are slowed down as they climb upwards on approach to stations, and sped up quickly on departure.

When did it go out of use?

The Royal Mail announced in 2003 that the railway would be closed by the end of that May. They had previously stated that using the underground rail was five times more expensive than using road transport.

Despite the Greater London Authority producing a report in favour of its continued use, the railway was closed in the early hours of May 31.

How was it revived?

In October 2013 the Postal Museum announced plans to open part of the network to the public. They were approved by Islington Council and work began in 2014.

A successful test drive was carried out through a section of the track last December and the public opening was set for summer 2017.

Two new battery-powered trains based on original designs but modified to carry up to 32 passengers were lowered onto the railway in parts by a crane through the original shaft.

How can I take a ride on it?

Launching on September 4, the Mail Rail will take visitors to the Postal Museum on 1km journeys that will last 15 minutes. The trip will begin and end at the Mount Pleasant sorting office station, on the site of the museum.

There will also be an exhibition giving insight into the former working rail line, featuring an original battery-powered locomotive used for repairs, an engineers’ tool set, bag exchange equipment and more.

What else is in the museum?

The new Postal Museum, opening on the earlier date of July 28, will feature interactive galleries, modern research facilities and a wide range of learning activities.

The museum aims to bring to life five centuries of the postal service, which it describes as the “first social network”.

The museum is located at the Mount Pleasant sorting office in Clerkenwell.

Tickets for both the museum and the Mail Rail can be purchased online.