Merthyr (Abernant) Tunnel

(Photos 1-10 © Sparhawk; photo 11 ©  and by permission of Rhondda Cynon Taf Libraries)

In 1845, Isambard Kingdom Brunel surveyed and prepared parliamentary plans for the Vale of Neath Railway which involved a 2,497-yard hole through the hill between Merthyr Tydfil and Aberdare, the second longest of Wales' tunnels. 650 feet below ground at its deepest point, it was built to accommodate broad gauge and formed part of the 6¼-mile Gelli Tarw Junction-Merthyr extension, opening on 2nd November 1853. It regularly takes two names - Merthyr, which is to its east, and Abernant.

The contractor (Mr Davis) sunk two construction shafts, 1,280 yards apart, from which he drove 7-foot headings. One of these shafts was 282 feet deep. A system of troughs and fans pumped air down to the face whilst a 1-inch diameter pipe forced water at high-pressure through ‘roses’ into the workings to clear the powder-smoke hanging in the air after blasting.

It was originally intended to accommodate two tracks and, at the west end, work was started to that effect. But after 300 yards had been excavated, the expense involved prompted the plans to be revised and the remainder was constructed for just a single line.

A third rail was added to the route in 1863, allowing its use by both broad and standard gauge stock, enabling GWR trains to reach Swansea. However the broad gauge rail was removed and the line converted to standard gauge on 11th May 1872.

In November 1877, the tunnel partly collapsed as a goods train passed through, almost burying the locomotive. This was attributed to the working of coal close to and beneath the bore, resulting in subsidence. The debris had been cleared and the tunnel prepared for reopening when a second collapse occurred, affecting almost 100 yards of the structure. Locals declared it to be dangerous, a claim unsurprisingly contradicted by the investigating officer. Mining continued to impact on the tunnel, resulting in part of it settling by 10 feet.

The S-shaped bore has masonry walls and a brick-lined roof. Close to its centre are two platelayers’ cabins. At the western end, for around 400 yards, the bore is wide enough for a double track layout; two small buildings mark the point where it suddenly becomes single.

The route was closed to traffic on 31st December 1962.

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