Before World War I, the streets of the largest city in Europe were choked with buggies and, increasingly, new inventions called automobiles.
Postal workers carrying the lifeblood of commerce and communication found themselves stuck behind stalled horses or marooned by the capital city's notorious fog.
So the city fathers concocted a solution for the growing mail crisis: Go underground.
Electric mail cars that rumbled under the streets of London wouldn't have to contend with blinding fog or traffic congestion. And with that, the Mail Rail system was born.
The 6.5-mile-long network of tunnels sits 70 feet below street level. Its heartbeat was electric-powered, driverless trains that shuttled mail beneath the city for nearly a century, before rising costs shut the system down.
"It was quite a remarkable thing for what they did in the 1920s," said Peter Johnson, a historian who has written about the mail rail. "They basically kept it going until the Internet killed it. People started using email."
Now, nearly a decade after the mail rail system was mothballed, it is being resurrected - but to shuttle people not parcels. Starting in July, the tunnels, tracks and replicas of cars will be used to take visitors to the National Postal Museum on a tour of the city's past.
As early as 1855, London leaders worried about mail delays began hatching plans for an underground network of parcel trains, but the costs in the 19th century always proved too high, according to the Postal Museum.
What would become the mail rail was the brainchild of a feasibility committee set up in 1909. Four years later, London lawmakers passed the railway bill, and construction began in 1914.
That was also the beginning of World War I, and although digging continued as England was sucked into the turmoil, metal at that time went to guns and ammunition, not railroad tracks. Thus, tunnels were dug, but tracks were not laid till later.
And the country found another use for a network of secret tunnels.
Mail rail construction resumed after the war, and the first segment opened in 1927, a few days before Christmas. By March, the rest of the mail rail was up and running.
Johnson's book described a massive, round-the-clock operation: "There were 90 cars in all, built by English Electric, and the practice was to run them as trains groups of three coupled together."
New cars and expanded stations came over the years, but the flow of mail rarely slowed.
During World War II, the mail rail slowed down when its stations were used as dormitories for troops. During Nazi Germany's blitz of London, the tunnels flooded several times, Johnson wrote. Still, the longest period of disruption was three weeks. Even then, the mail simply traveled by road.
According to Johnson, mail rail traffic hit a peak in 1962, but even two decades after that, a Post Office survey found the rail could still deliver letters for 40 percent less than the cost of moving letters by road.
By 2003, that equation had flipped. Roads were expanded and improved, Johnson said. And transport by truck was more economical. The mail rail system was shut down.
For more than a decade, the rails and tunnels have remained silent, attended to by a team of three engineers.
That changes in July, when the mail cars will be put back in service.
But instead of ferrying letters and parcels, they'll carry tourists who, as Smithsonian Magazine put it, "will be able to ride through these abandoned tunnels for the first time in the railway's history."
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)