Cefn Glas Tunnel carried the Taff Vale Extension of the Newport, Abergavenny & Hereford Railway which was absorbed into the Great Western’s empire in 1863, a year before the line opened. The route made an end-on connection with the Aberdare Valley Railway at Middle Duffryn Junction; the southern end was at Quakers Yard, named after a local burial ground. The single track route closed on 15th June 1964.

The 703-yard tunnel is straight except for a slight northerly curve close to the western portal. It is amply equipped with refuges and its profile changes at various points. Although mostly dry, the tunnel is not in great condition - towards its centre, sections of its lining have come away. Comprising a brick roof with masonry walls, its patchwork nature suggests that, even during operational times, much repair work was needed.