The existing four-mile stretch already uses part of the railway from Harrogate to Ripon, which closed in the 1960s, including a viaduct over the Nidd Gorge.
And the extension would follow the path of a single-track branch line which was already long-mothballed by the time the last train ran to Ripon.
The Nidd Valley Railway left the Harrogate-Ripon line at Ripley Junction and ran for 11 miles with station stops at Ripley Valley, Hampsthwaite, Birstwith, Darley and Dacre, terminating at Pateley Bridge.
Passenger services on the line peaked in 1927, with seven trains per day in each direction, but by the 1950s this had dropped to two.
Its success partly depended on its connection to the Nidd Valley Light Railway, which between 1907 and 1937 transported workers and materials to the reservoir construction projects at Angram and Scar House in Upper Nidderdale. When these were complete, both lines suffered a decline in traffic.
Competition came from local buses which called at more villages, and by 1951 the passenger service had been axed. The line remained open for goods until 1964.
The stations at Pateley Bridge, Dacre and Hampsthwaite survived and are now private houses.
Many parts of the old trackbed remain as footpaths, and although landowner permission may be required, it is feasible that they could be upgraded for cycle use.
Campaign groups have pointed out that the railway route is flat, following the River Nidd, and offers a more attractive alternative to the main roads and hilly stretches between Harrogate and Pateley Bridge currently in use.
The current Nidderdale Greenway from Harrogate to Ripley opened in 2013 and has been hailed as a resounding success, with plans to extend it along the route of the main line to Ripon having also been debated.
A greenway to Scar House would pass through Pateley Bridge, boosting tourism in the town, while also offering a route for commuters.
Harrogate Council are now officially backing the Sustrans-led scheme and a steering group is expected to be formed soon. The full route could be open within five years.