The Blaencwm tunnel, closed for nearly 50 years, could reopen after a campaign by the Rhondda Tunnel Society.
The society wants to reopen the route, which runs for 3km (1.8 miles) to Blaengwynfi, Neath Port Talbot, for cyclists, walkers and tourists.
The tunnel was closed during cutbacks of the UK railway network in the 1960s.
In May this year, the Welsh government said it would commission a study to look at reopening the tunnel for tourism.
The first photos from inside a disused Rhondda railway tunnel, which campaigners want to convert into a cycle route, have been released.