(Photos 1-3 & 5-7 © Rick Garside, photo 4 © KevinTrahar)
Authorised by Parliament on 3rd August 1846, the Vale of Neath Railway provided a Brunelian broad gauge connection between Neath and Merthyr Tydfil. The western section, as far as Aberdare, was opened on 24th September 1851 and the completed route was consumed into the Great Western's empire 14 years later. In 1863, a third rail was added to provide a mixed gauge route, with a complete conversion to standard gauge taking place on 11th May 1872.
Heavy engineering was demanded on the VoN, including two tunnels at Pencaedrain and Abernant/Merthyr. Pontwalby was the site of a viaduct which launched the two-track line over the Gwrelych valley. Built by contractors Gale and Warden, it comprises five spans of 35 feet each, giving an overall length of about 80 yards. The tracks enjoyed a falling gradient to the west of 1:50 as they were carried 80 feet above the stream.
Whilst the arches are formed in red brick, the rest of the structure was built in locally-quarried grey stone. The piers incorporate arches to reduce weight and focus forces onto the foundations. The westernmost span is blind, with a wall erected across it at the south side. This features a circular opening.
Today the deck is tarmacked, hosting a foot and cycle path. The viaduct received a Grade II listing in September 2003, but you can hardly see it for all the encroaching foliage and the ivy climbing up the piers.
(KevinTrahar's photo, taken from Geograph, is used under this Creative Commons licence.)