(All photos © Andrew King)

Bored through Ordovician shales, the single-track tunnel south-west of Pencader - also known locally as Alltwalis or Dolgran Tunnel - was built to Brunel's broad gauge. It formed part of the Llanpumsaint-Pencader section of the Carmarthen & Cardigan Railway, taking the line between the valleys of Skanda Vale and Dolgran.

Brunel was one of several engineers to survey the line although his was not the design finally adopted, this being longer to provide easier gradients. At 988 yards in length, the tunnel is straight for most of its length before starting to curve south close to the more northerly of two construction shafts, later used for ventilation purposes.

Work got underway in the spring of 1857, with the heading being driven from both ends and two shafts, creating six working faces. In 1859, to help reduce costs, the C&C applied for Parliamentary permission to convert the route from broad to standard gauge. This was turned down.

Excavations were completed in March 1861, although construction of the railway either side of the tunnel continued for almost three years. The southern approach involved numerous cuttings and embankments.

The Board of Trade inspected the structure in January 1864 with the first train passing through on 1st April 1864. The construction costs bankrupted the C&C which gave way to the Great Western in 1881. Conversion to standard gauge had occurred nine years earlier.

The withdrawal of passenger services came in 1965 but the line continued to carry freight until 28th September 1973.

The tunnel is muddy at both ends but the majority of its interior remains dry thanks to the natural drainage resulting from its position at the line's summit. A tiny aqueduct takes a water course across the western end of the south portal. The brick lining is in very reasonable condition. Close to the northern shaft, a side passage - reputedly built as an emergency escape route - heads horizontally to the surface.