In 1956 the British Electricity Authority compulsorily acquired the Ffestiniog Railway (nprn 34660) line above Moelwyn Tunnel (nprn 34938) to construct a reservoir and hydroelectric power station near Tanygrisiau (nprn 401209). A new route was surveyed, involving a spiral loop around Dduallt Station (nprn 309125) in order for the line to gain height and then pass through a new tunnel (nprn 407723) and along the west side of the reservoir before regaining the original alignment at Tanygrisiau Station. The Dduallt spiral was completed in 1971 and the new route to Tanygrisiau opened in 1978.

B.A.Malaws, RCAHMW, 22 May 2008.