A bridge, constructed in brick, carrying the Penrhyn Railway over the Bethesda branch of the London & North Western Railway.
The Penrhyn Railway was built in 1879, to a gauge of 1ft. 11ins. (0.58m.) and incorporated parts of the 1801 Penrhyn Railroad, although this section was entirely new build. It was built to carry slates from Penrhyn Quarry to Port Penrhyn, near Bangor. The standard-gauge L.&.N.W.R. Bethesda Branch was crossed in a deep cutting, just east of the latter's Tregarth station, on this single span skew arch, erected at L.&.N.W.R. expense.