The longest underground cycle route in Europe is set to connect two Welsh valleys using a neglected Victorian railway line.
Campaigners have finally seen light at the end of the tunnel, after the two-mile track was opened up for the first time in 50 years.
The Rhondda Tunnel once hosted coal trains that rumbled 1,000 feet below the mountains, from the mines of Rhondda to the ports in Swansea Bay.
The tunnel was closed as part of the Beeching cutbacks in 1968, when the UK’s rail network was restructured and reduced.
Surveyors said in January that they were confident the tunnel could be restored.
A concrete seal was removed from a vent shaft on the tunnel in April, as part of a widely-supported project to transform it into a cycle route.
On 5 May workmen successfully broke through a 2.5m wall of solid concrete in the structure after a week of drilling.
Engineers will now inspect the site to produce an estimate of repair costs.
Members of the public as well as the Welsh actor Michael Sheen have got behind the plans.
Inspectors must first carry out tests on the structure, which have been backed by a £90,000 community grant.
Highways England, which owns the site, is willing to hand the tunnel over to be developed into a cycle path.
Rhondda Tunnel project secretary Tony Moon told Wales Online: “This is certainly a fantastic day for us.
“We are progressing steadily in the right direction and hope that very soon after the report from the detailed examiners (Balfour Beatty) the tunnel will come out with a clean bill of health.
“This will then give the all-clear for either the Welsh government to take ownership of the tunnel from Highways England, or perhaps a joint share with Neath Port Talbot council, Rhondda Cynon Taff council and the Welsh government.”
If the project is completed the Rhondda Tunnel will be the longest underground cycle route in Europe, and the second longest in the world after the Snoqualmie Tunnel in Seattle.