The Cambrian lines had two notable timber viaducts in Gwynedd, north west Wales. Both were constructed under the supervision of a succession of the railway company's chief engineers — Benjamin Piercy (1827-88), Henry Conybeare (1823-84) and George Owen (c.1827-1901). Such structures were once common all around Wales, and a typical feature of its coastal railways. Most have long been replaced.

The railway viaduct known as Pont Briwet was constructed in 1865-67. It carried the Cambrian Coast Railway and a single-lane toll road on 22 equal spans of 5.8m across the River Dwyryd, near Penrhyndeudraeth, Gwynedd. In 1932, the viaduct was substantially rebuilt, but ultimately it proved unable to bear modern traffic loadings. In February 2012, Pont Briwet was approved for demolition by Snowdonia National Park, and was replaced with a new road and rail crossing in 2014-15.

By contrast, Barmouth Viaduct, also completed in 1867, survives and remains in use. Ongoing maintenance and replacement of timbers affected by borer has kept it operational. It carries a single track of the Cambrian Coast Railway and at 699m long is one of the longest extant timber viaducts in the UK.

Timber rather than iron was chosen for its construction because it could be brought to site by sea, directly from the Baltic, and was in this case around a quarter the cost of an iron structure. It was also thought — incorrectly — that the wide sandy estuary of the Afon Mawddach was borer free. The viaduct is composed of 113 timber spans, two spans on cast iron piers and an iron drawbridge over the navigation channel, since replaced in steel and no longer operational.

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