The Merionethshire Railway (MR) was a proposed 1 ft 11+3⁄4 in and 4 ft 8+1⁄2 in dual gauge railway in south Caernarfonshire, North Wales, United Kingdom. It was incorporated by an act of Parliament, the Merionethshire Railway Act 1871 on 29 June 1871. Work to build the line never started, though parliamentary extensions of time to do so were obtained by the Merionethshire Railway Act 1876 and the Merionethshire Railway Act 1879, and the Merionethshire Railway Act 1882. Powers to build the line lapsed in 1885 and were abandoned by the passing of the Merionethshire Railway (Abandonment) Act 1887 on 12 July 1887.