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
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.
"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."