In 1913 a railway to carry construction traffic was built along the bottom of the Grwyne Fawr valley as the road NPRN 267107 along the side of the valley could not cope with the quantity and type of goods being transported. The railway ran southwards from the reservoir to the temporary village at Blaen y Cwm for the reservoir construction workers and their families and beyond. This NPRN has been given to the section from SO2533028276 to SO 2267031084.
The railway went over the foundation of the dam wall by a 18m trestle bridge. As the dam wall was built upwards, the railway was taken through a tunnel through the wall. The War Dept requisitioned the track in February 1917 and by April 1917 the track had been lifted completely on this section of railway. It was re-laid by November 1919. In 1927, the tunnel through the dam wall was filled in and a new piece of railway, NPRN 267324 was built to access the quarry at the northern end of the reservoir. The entire railway was lifted when the reservoir was finished in 1928 Source: Tipper Rev DA, 1975, Stone and Steam in the Black Mountains.
When visited in September 2007, no trace of the railway could be seen, its route now covered with tarmac to provide a road to the bottom of the reservoir.
J.J.Hall, Trysor, 22 January 2008