Originally, when being proposed and built under the North Wales Narrow Gauge Railways, the main line was to a place called Bryngwyn, and a secondary line was to South Snowdon (Rhyd-Ddu). This was the point the lines diverged. The 'main line connected to a number of slate quarries on the slopes of Moel Tryfan, the upland area which rises to the right of the railway as viewed from a train heading from Dinas Junction towards Waunfawr and turned through 180 degrees to run behind the station and back in the opposite direction to the main line. The gradient in this section started as 1 in 39 but eased down to 1 in 49 as Rhostryfan Station (3/4 mile) was reached. The station consisted of a stone station building, a signal box and one siding with a goods shed at its end.
Leaving the station, the line entered a cutting and then crossed a small stream before passing under the main road through a village and within 50 yards crossed a minor road on the level to head up through open countryside once again on slight embankments and cuttings to enable it to use the natural contours of the hill. The line then crossed the Bryngwyn road on the level and immediately the gradient becomes 1 in 39 as it entered another cutting on a 180 degree curve emerging on an embankment. It then curved back the other way, round Bryngwyn Farm, and crossed the Bryngwyn road again on the level. Immediately across the road lay Bryngwyn station (2 1/4 miles). Here was a signal box beside the level crossing gates, a stone station building, a goods shed, a run round loop and a siding. Set someway away from the station stood an explosives store.
For the passengers, this was the terminus of the branch, but beyond the station stood the incline and the real purpose of the branch. The incline was of double track and rose from 650ft above sea level to 895 ft over its half mile length. Unlike most similar railways, the Welsh Highland owned the incline up to the drumhead and the private tramways did not start until beyond that point. Here four branches converged, leading off to the Alexandra Slate Quarry, the Moel Tryfan Slate Quarry, the Fron Slate Quarry, and the Braich Slate Quarry. Later, a long connection was added to Cilgwyn Quarry, via an extension of its existing tip line to the "Cilgwyn Horseshoe" near Carmel and Y Fron; this was the only direct connection between the WHR and one of the main group of Nantlle Vale quarries. Looking back from the top of the incline, the line's starting point at Dinas could be seen some 2 miles away whilst the line had covered over twice that distance in reaching here.
The branch trackbed as held by the Official Receiver passed into FR ownership together with the remainder of the WHR land. The railway has no plans to rebuild the branch, and has cooperated with local partners (primarily Gwynedd Council and Llanwnda Community Council) over use as a footpath, provided nothing is done which would prevent reinstatement of a railway at some later point. The branch trackbed between Rhostryfan and the first crossing of the Bryngwyn road was cleared and opened as a footpath in 2002, with interpretative signage added in 2006 in cooperation with the Uwchgwyrfai Commons project. Discussions continue over opening the largely overgrown trackbed from Rhostryfan down to Tryfan Junction as an extension of the path, linking it and Rhostryfan village with the WHR, and over leasing the trackbed to Gwynedd Council; some delay has been caused by an adverse possession claim affecting part of the section.
A larger, photo enriched, description appears on the official WHR site here (external Link)