Work is under way on a new cycle path between Plymouth and Tavistock.
The 17-mile (26km) route will follow an old Great Western Railway branch line through a tunnel at Grenofen built by Isambard Kingdom Brunel.
It will also include a £2m, 1,000ft (305m) long viaduct across the Walkham valley, which is currently being built.
The full route, which will form part of the Drake's Trail walking and cycling network, was expected to open in Spring 2012, project managers said.
'Critical missing link'
The Grenofen tunnel, bought by Devon County Council for £1, was built in 1859 for Great Western trains. It closed to services about 50 years ago.
Project manager Graham Cornish said its being part of the route would give the tunnel a new lease of life.
He said: "Most of the stone work is in pretty good condition.
"The tunnel is an absolutely critical missing link in this."
The trail is being provided by Devon County Council, with funding from the European Union and the South West Regional Development Agency.
West Devon Borough Council and the Devon Renaissance regeneration company are also involved with the project.