Built by Baldwin Locomotive Works of Philadelphia in 1918 (possibly Builder's number 49604), this locomotive was an 0-4-0PM built for service in Word War I with the French Army. It was purchased in February 1925 for use on the Festiniog and also the Welsh Highland through Honeywell Brothers, the agents used in the purchase of the Simplex - at what seemed to be a bargain £248 13s 4d. Put to work shunting at Minffordd and together with the Simplex it was used for a short time to replace horse traction on part of the Croesor Tramway. It proved too heavy for the light track there and was returned to shunting duties. The need for economical winter services led in 1928 to the Baldwin being fitted with vacuum brake for service on the Welsh Highland. Although no records are known to survive as to its actual use it probably was used on the WHR and it was certainly used as a rescue engine for passenger trains on occasion.
However the Baldwin had feet of clay. Probably unbeknown to the Festiniog the American Society of Civil Engineers had in 1920 severely criticised the design. The actual petrol motors, mostly built by the Pittsburgh Model Engine Co, needed frequent repairs to the point where the Army engineering regiments asked for new motors. There were also problems with clutch and gear cases. The poor spring design was also criticised as were the long overhangs which made its riding lively at service speeds .The Baldwin had became much worn and broke an axle in April 1929. It was criticised for heavy fuel consumption in the autumn and had a heavy motor overhaul that winter. Not as popular as the Simplex it seems to have been increasingly confined to the shed after Stephens death.
The original engine was beyond repair by 1954 and in 1956, Moelwyn was re-engined with a Gardner diesel. Carrier wheels were added at the front in 1957, making Moelwyn a 2-4-0DM and a more stable runner. It became a valuable mainstay both on engineering trains and as a back up passenger locomotive.
The locomotive returned to the FR in August 1998 after a lengthy restoration in England, and now forms part of the Railway's heritage fleet. The name was derived from a typical FR play on words, "Moelwyn" being the name of the mountain rising north of Tan y Bwlch; Moel means bare hill, but can be read as bald, so FR "Welsh" gives the translation of Baldwin!
In 2009 Moelwyn entered Boston Lodge works for a major overhaul, including work to its motion, engine and wheels as well as a re-paint in FR Maroon, after many years in green. It has been fitted with a new pony-truck following an appeal by the Ffestiniog Railway Society.