Enticed by a slice of the local slate action, the London & North Western Railway drove its 4¼-mile single-track branch to Bethesda, climbing over 300 feet from its junction with the North Wales main line. Authorised in 1880, it opened to passenger traffic on 1st July 1884 and goods 14 months later. Edward Walter Nealor Wood engineered the route. It featured a couple of small viaducts and a 297-yard tunnel, just to the east of Tregarth where one of the line's two intermediate stations was sited.
Heading towards Bethesda, the line passed through a short tunnel of 27 yards (sometimes known as Tregarth No.1) and entered a sheer-sided rock cutting. Embedded at the end of this is the impressive masonry portal to the main tunnel. This features a single ring of oversized voussoir stones.
Short sections at both ends are entirely lined in engineering brick but the intermediate majority is formed of near-vertical masonry side walls - into which refuges are cut - and an eliptical brick arch. A drain is buried beneath the Down side of the tunnel floor whilst the Up-side wall accommodates signalling pulleys on wooden plates. Tablets are also provided at intervals of one chain.
Though straight for much of its length, the tunnel curves to the west close to the north entrance. The southernmost 12 feet is effectively a bridge - with an associated change in profile - carrying a lane over the alignment. But this lane was once the trackbed of the 1'10¾"-gauge Penrhyn Quarry Railway, also built to carry slate to a nearby port for transhipment.
The skewed south portal is a more frugal affair, fashioned only from brick. Immediately beyond it, the trackbed is launched over the River Ogwen on a neat but diminutive viaduct which features a main span over the water and three small arches at both ends.
The tunnel saw its last traffic on 7th October 1963 although passenger services ceased in 1951. It is now an occasional party venue! The northern section of the branch's trackbed already hosts a footpath and plans are being developed to extend it through the tunnel to reach Bethesda.