The Pontarddulais Tunnel is the oldest surviving railway tunnel in Wales, dating from circa 1839, the era of the horse-drawn tram. It was lowered at the invert to accommodate the standard-gauge-line. The tunnel is 72 metres in length and about 4 metres in width. Construction is in masonry, with a segmental arch supported by vertical walls. Apparently there have been long standing drainage problems arising from the lowering of the invert and in 1984 a new concrete bed and drains were laid through the tunnel.
SOURCE: HEW 1389. Records at Paddington.
(Site entry by Stephen Hughes for Buildings of Glamorgan, John Newman, 1995)
