On the right hand side of the Heads of the Valleys Road on the final approach to Brynmawr is a section of an early and important tramroad, including an impressive causeway section with a stone bridge – the Hafod Arch.
The Clydach Railroad was built under powers provided by the Brecknock and Abergavenny Canal Act, and connected coal and iron ore mines, limestone quarries and ironworks at the head of the Clydach Gorge with the canal at Gilwern. It was designed by the canal company engineer, John Dadford. Work began in 1793 and was completed in the following year. It was constructed with a gentle gradient for the whole length of the Gorge, so that waggons could descend by gravity with brakes and be hauled back by horses. This was distinctive from tramroads built shortly afterwards which had level sections linked by inclined planes. Iron edge rails were used, on both iron and wooden sleepers.
The railroad fell out of use gradually during the middle of the nineteenth century as traffic went instead by the Monmouthshire Canal's tramroads to the south. The tramroad probably closed in the 1860s. The tramroad is cut into the rockface on the uphill side and embanked on the downhill, but the central section forms a stone revetted, earthen causeway over the Nant Melyn. This is built of rubble sandstone blocks, with dressed quoins and voussoirs and patches of dressed replacement stone. One tall stone arch crosses the stream, partly cut through solid rock, with a small flood relief arch to its west. The bed of the tramroad is grassed over, and it is not certain whether any sleepers or rails may survive below ground. The parapets to the causeway are in cock and hen construction.