Enticed by a slice of the local slate action, the London & North Western Railway drove its 4¼-mile single-track branch to Bethesda, opening on 1st July 1884. It featured a couple of small viaducts, a 297-yard tunnel and two intermediate stations.

At Glasinfryn, the track was carried over the River Gegin by a viaduct comprising seven arches of 30 feet either side of a 60-foot skew span over the water. The structure incorporates a curve to the east of approximately 24 chains in radius.

Adding greatly to the local landscape – hence its Grade II listing earned in September 1997 – the piers, spandrels and parapets are built from red sandstone. Imposts serve as the base for the brick arches which are generally formed from five rings of blue brindles, except the skew arch which has seven rings. Either side of the river span are wedge-shaped piers, each incorporating three buttresses. These form refuges at deck level.

Redundant since 2nd October 1963, the viaduct had a Grade II listing bestowed upon it in 1997. It now plays a role in the 11-mile Lon Las Ogwen Railway Path, managed by Sustrans.