Combe Down tunnel was built as part of the Bath to Evercreech line
Work has begun on a £1.9m scheme to create a four-mile long cycle and walking path between Bath and Midford along a disused railway line.
The project will take people under Bath's streets through the derelict Devonshire and Combe Down Tunnels.
The project is being largely financed by transport charity Sustrans and Bath and North-East Somerset Council.
Sustrans' Malcolm Shepherd said: "It's a very ambitious project, but it's very popular and will be heavily used."
The tunnels were part of the Somerset & Dorset Railway line to the south coast, but they have lain unused since the 1960s.
The idea of the cycle route from the city centre to nearby beauty spots has been lobbied for by the Bath Two Tunnels campaign group for four years.
Frank Tompson of the group told BBC Points West: "Like all the best projects it was dreamed up in a pub - the Raven in Bath - with nine people who thought it was a good idea."
Another campaigner, Gitte Dawson added: I am excited that we are finally putting the first spade in the ground.
"Lots of people have done a great deal of campaigning, fundraising, planning and publicising to get to this point."
The work has begun clearing a northern entrance into the Devonshire tunnel, in Linear Park.
Sustrans says the path should be in use by next year.