The old Moelwyn Tunnel (nprn 34938) on the narrow-gauge Ffestiniog Railway (nprn 34660) was cut in the 1950s by the British Electricity Authority during construction of a reservoir and hydroelectric power station near Tanygrisiau (nprn 401209), sealed up and abandoned. A new tunnel, some 250m long, was excavated to the west to carry the re-aligned railway, known as the 'Deviation', between Dduallt (nprn 309125) and Tanygrisiau stations. The deviation also included a spiral loop (nprn 305758) around Dduallt Station. Completed in 1977, the new tunnel was cut through poor quality rock and the interior treated with 'shotcrete' to stabilise the bore.
B.A.Malaws, RCAHMW, 22 May 2008.