Two wooden Brunel fan viaducts, dating from 1854/5, were built on the broad-gauge Dare and Amman Branch of the Vale of Neath Railway. The larger of the two, Gamlyn Viaduct, 183m long and 21.3m high, spanned the Afon Cynon some 2km east of Hirwaun. The line closed in 1939 and the viaducts were dismantled in 1947, except for the masonry piers, which also survive at the smaller Dare Viaduct (nprn 34797), 2.5km to the south.
(Source: A Guide to the Industrial Archaeology of South East Wales, AIA, 2003)
In 1941 both viaducts were surveyed and drawn by the Great Western Railway. A photograph of one of the viaducts in the 1920s is reproduced in: John Binding, Brunel's Cornish Viaducts, Atlantic 1993.
B.A.Malaws, RCAHMW, 28 January 2009.

Originally, six cast-iron Great Western Railway boundary markers flanked the viaduct: two at the northern end (SN98190501-SN98170501), two near the river-bank (c.SN98180495), and two mid-way (c.SN98180498).
In February 2017 the northern-most markers were still intact, one of the river-bank markers had been removed, and the middle pair beset by impenetrable brambles.
Source: Enquiry No. RC17-0033
LM, RCAHMW, 3rd February 2017