London's secret Mail Rail is turned into a first-class tourist attraction

The Mail Rail attraction allows visitors to travel through the tiny hidden tunnels under London. Photo / Jack Taylor, Getty

You've heard of the Tube but there's another, more mysterious network beneath London's streets with a very different purpose, writes Benedict Brook for News.com.au.

Anyone who's been to London will be familiar with the Underground.

Every day, up to five million people — commuters and visitors alike — crowd onto its surprisingly small trains, to navigate their way around the UK capital.

The Tube's network of tunnels creates a dense, spaghetti-like web beneath the traffic-choked streets.

But, there is another, altogether more mysterious network of tunnels under central London. Away from prying eyes, this parallel underground system was never intended for commuters. This is the Mail Rail.

Eerie and abandoned for decades, the subterranean world lay in darkness and dust, as quiet as the grave, yet almost within touching distance of busy Londoners.

"If you head into these tunnels near Oxford Circus you're only 5cm from the Underground station. It's so close so you can actually hear people talking on the platforms," Harry Huskisson from the UK's Postal Museum tells news.com.au.

Reopened as part of a $45 million revamp of the Postal Museum, the unassuming entrance of Mail Rail lies beneath the vast Mount Pleasant sorting office close to London's financial district.

Descending a short staircase, you're delivered to another world far from the teeming trucks, full of parcels, up on street level.

Once the maintenance depot for the network, it's now the jumping off point for this almost forgotten system.

"It's the only railway of its kind that we know of and it was incredibly ahead of its time when it was built," Mr Huskisson says.

"Completely driverless, it ran on electric power 22 hours a day while there were still horse and carts on the streets above."

At its peak, four million letters a day were funnelled underground taking countless vehicles off the streets.

The Mail Train was head of its time: completely driverless, it ran on electric power 22 hours a day while there were still horse and carts on the streets above. Photo / SSPL, Getty

"The post would be sorted, put onto Mail Rail and then sent to Liverpool Street or Paddington main line railway stations and then onto ports and as far away as Australia."

Some bizarre pieces of mail were transported.

"In the 1930s you were able to carry almost anything by post. There are pictures of vans loaded with live fish and bees, scorpions even," Mr Huskisson says.

Mail Rail gladly transported dead animals — there were lots of slaughtered rabbits, but senders had to guarantee "no liquid was likely to exude".

There's a sudden clatter and a metallic screech and the new, specially built, passenger train (there were only freight carriages when the system was open) begins its descent into the murk, down to 20m underground.

As the carriages sink deeper under London, oily black and silver tracks veer off left and right towards dark nothingness. Momentarily a "graveyard" is spied below — where rusting trains of old have been parked, never to carry letters or see daylight again.

The vehicle slides beside a platform where mail was once loaded and unloaded.

More of the history of this odd piece of infrastructure is relayed, including the vital role the Royal Mail, Britain's postal service, played in World War II, keeping the lines of communication open across the ravaged continent.

The trip is not for the claustrophobic. The tunnels were never designed for human cargo and there's not much room for manoeuvre. It makes the regular Underground feel positively cavernous.

"Cosy, is what I like to call them," remarks Mr Huskisson about the trains.

As ahead of its time as it was, in the 2000s Mail Rail fell foul of progress. The Royal Mail was consolidating its offices into fewer, larger facilities and it didn't need an expensive rail line connecting only a few CBD hubs.

Its compact size didn't help either. "More and more, parcels rather than letters were being delivered and you couldn't get as much on the trains," Mr Huskisson says.

From hundreds of staff, the system went to just three engineers who kept the mothballed system shipshape in case it was needed again — which it finally was when the Mail Rail opened for visitors late last year.

There were various suggestions of what to do with the tunnels in the interim. The lack of light would be ideal for growing mushrooms. One fanciful plan was for it to become a subterranean cycle superhighway. (Except it's likely riders would have scraped their heads on the shallow tunnel roof.)

Parts of the vacant tunnels have been used for surveying work in preparation for London's flash new $25 billion Elizabeth Line underground commuter rail system due to open in 2019.

Being so inaccessible, silent and abandoned, Mr Huskisson says there were always rumours the Mail Rail might host some unusual — perhaps otherworldly — visitors.

"We asked one of the engineers, Ray, who worked on the line for 30 years, if there was anything untoward down there.

"Ray said he had seen a fox that lives in the tunnels but he was adamant there were no ghosts.

"But when he first met his future wife, she said her house was haunted — that in her basement, she would hear noises and shaking," he said.

"Ray looked into it and realised her house was on the Mail Rail route — his wife had been spooked by the noise of the trains. So, while there are no ghosts on the Mail Rail, the Mail Rail has itself been mistaken for a ghost."

Mail Rail is part of London's Postal Museum, in Clerkenwell close to Kings Cross and St Pancras stations.