Barmouth Railway Viaduct was originally a timber viaduct built in 1867 to carry the Cambrian Railways coast line over the Mawddach estuary. At 243 metres (800 ft) long it was the longest timber estuarine bridge in Britain. The original iron rolling centre section was replaced by a steel lattice swing bridge in 1899. The later structure has two hogback trusses on cylindrical piers, one of which is a swing span of 136 feet (41.5 metres) and the other is a fixed span. The wooden bridge was replaced by 113 openings of eighteen feet (5.5 metres) span on timber pile trestles which are now encased in glass-fibre reinforced concrete sleeves.

Claire Parry, RCAHMW, 14 June 2011.

Sources:
Hague, D.B (1984): 'A Guide to the Industrial Archaeology of Mid-Wales'. Association for Industrial Archaeology.
David Gwyn & Merfyn Williams (1996) `A Guide to the Industrial Archaeology of North West Wales?. Association for Industrial Archaeology.