The Barry Railway Company was born to release the stranglehold of the Taff Vale Railway and Cardiff Docks on the export of South Wales’ coal. Work on it started in 1885 and thanks to their efficiency, by 1910, Barry Docks had overtaken their near neighbour in terms of tonnage shifted.

Within four years, the company had built a substantial rail network including several branches and an 18½-mile main line from Trehafod into the docks. Included in this was a double-track bore of 1,323 yards at Pontypridd which first saw active service in 1889. Seven years later, a station was established at its northern entrance; two years after that, another opened close to its southern end. The pair provided alternative names for the tunnel – Graig and Treforest.

Trehafod-bound trains curved slightly to the north on their final approach to the tunnel. The west-side retaining wall extends upwards to become the portal’s stepped wing wall. The horseshoe-shaped bore is straight, with masonry side walls and a brick arch. Occasional refuges are provided. As the line emerged into Graig Station, the profile becomes lower to pass beneath High Street.

Like its sibling, the north portal is stone-built. It incorporates an inscription stone that records the tunnel’s 1888 completion date, together with the names of its engineer and contractor – James W Szlumber and John Mackay respectively.

Whilst the two stations closed in 1930, the tunnel continued to see traffic until June 1951. Today it is sealed at both ends and largely flooded in-between. The site of Graig Station now serves as a hospital car park; the same fate has befallen the southern approach cutting, benefitting the University of Glamorgan.

In 2011, the tunnel’s northern blockwall was replaced with gates as a precursor to maintenance work being carried out. Its owner, British Railways Board (Residuary), is likely to shotcrete the lining close to both portals.