Travel underneath London on the Mail Rail

London's newest tourist attraction is a tiny underground train that used to carry mail for the postal service. Before it opens to the public in September, CNET went for a ride.

Speeding beneath London in a metal tube isn't new: Londoners have been doing it since the first Underground line opened in 1863. But there's another underground railway stretching across the city that few people have ever seen, until now. 

Opening September 4, "Mail Rail" is shortened version of a hidden railway network that stretched six-and-a-half miles across London, from Whitechapel in the east to Paddington in the west. From 1927 to 2003 when it was abruptly and quietly shut down, tiny trains carried letters and packages between eight Royal Mail post offices for fast delivery under the capital's traffic-choked streets.

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Following its closure, Mail Rail's tunnels and equipment lay largely untouched until 2011 when postal service employees opened the network again for exploration. Converting the network to carry passengers began five years later, a century after the first tunnels were dug.

When Mail Rail opens to the public next month (you can prebook online now), you'll be able to take a 20-minute ride on 1 km of circular track. It's a tight fit inside the trains, but you're afforded a great view outside the clear canopies that fold over your head. 

Along the way you'll pass through two former stations where crews would sort mail and load it onto trains. You won't be put to work, but you will view a film of the Mail's history broadcast onto the station walls. After your ride, there's a cool exhibit of old trains and equipment.

Mail Rail is located in London's Clerkenwell neighborhood across the street from the renovated an expanded Postal Museum. Admission to the museum and a ride on the railway is £11 (about $14.50 or AU$18) for adults and £8 for kids.