0.6 miles (1km) of Mail Rail's 23 miles (37km) of track will open as a tourist ride
Plans to open The London Post Office Railway - known to many as Mail Rail - as a tourist ride have been approved by Islington Council.
Visitors will be able to ride 0.6 miles (1km) of the tunnels under central London from 2020.
A new postal museum will open at Mount Pleasant, in central London, in 2016.
The British Postal Museum & Archive (BPMA) still needs to raise £0.5m and plans to launch a public appeal later this year.
The team also expects a decision to be made in May on its application for £4.5m funding from the Heritage Lottery Fund.
Mail Rail was approved by an Act of Parliament a century ago, and during its heyday its driverless trains carried 12 million postal items daily on the line stretching from East End's Whitechapel to west London's Paddington.
In its prime, 220 people worked on the line, which runs beneath Oxford Street in central London - at one point within a few feet of the Bakerloo Line.
But by the 1990s, Royal Mail built a new hub in Willesden, west London, and by 2003, only three of eight Mail Rail stations still worked
Tourists would get on to a train in Mount Pleasant's depot and descend into the tunnels
Mail Rail The tunnels are narrower than Tube tunnels at just 7ft (2.1m) in diameter
That year, Royal Mail said the line cost five times as much as using roads and the network was mothballed.
Ray Middlesworth, who has worked as an engineer in the tunnels for 27 years, said: "It's the holy grail for underground explorers - a hidden part of the rail network.
Explore the darkness of London's Mail Rail with engineer Ray Middlesworth. Video at link by Dan Curtis.
"Some people called it the Post Office's best-kept secret."
BPMA director Adrian Steel said: "It is a fantastic opportunity that Islington borough council has given us - the green light to open up these unique tunnels to the public and reveal the captivating story of Mail Rail."
Up to 220 people worked in the tunnels
The trains are 4ft (1.2m) high and 29ft (8.8m) long
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Analysis
Andy Dangerfield BBC News, London
I was lucky enough to have the chance to take a ride on a Mail Rail train specially converted for passengers.
At Mount Pleasant, you enter what feels a lot like a normal Tube platform, but the tunnels at each end are smaller, reducing to 7ft (2.1m) in diameter.
A tiny train sits on the track, with a dozen small seats squeezed into the carriage. Once inside, a plastic shutter is secured over your head and it feels like you are about to embark on a rollercoaster ride.
It sets off and you notice every bump as it rumbles into the tunnel and hear the brakes screech as it winds around tight corners.
You get within inches of the filthy tunnel walls and limescale stalactites hanging from the roof.
Tourists will get the chance to experience the thrill of this for themselves from 2020.