This 4wPM locomotive of around 20hp was constructed by Muir Hill (Engineers) Ltd in about 1922. It came to the FR on loan from the Aluminium Corporation's Cowlyd Reservoir Railway near Dolgarrog, arriving at Blaenau Ffestiniog on 9 April 1924. The loan was arranged by Henry Joseph Jack, who at the time controlled the WHR and FR and believed that a fleet of these units could provide cost-effective haulage for most purposes.
Based on a Fordson tractor, the Muir Hill had three speeds in forward but only one in reverse. Trials were conducted in early May. It performed about as well as could have been reasonably expected, being barely able to pull 'an ordinary guards van' from Beddgelert to South Snowdon (Rhyd Ddu). Later in May it was tried to power the machinery at Boston Lodge. In a further trial it ran very well from Blaenau Ffestiniog to South Snowdon with a small van but could only return to Beddgelert at 2¼ mph because of its inadequate reverse gearing. These poor results were reported to the Board in June and the unit returned to Dolgarrog in August 1924.
See the Colonel Stephens site for a picture, though this is actually a similar machine used on the Ashover Light Railway in 1943.
Another similar tractor (but fitted with a cab) was used by the quarry companies in the period between 1946 and 1963 when they worked their own traffic over the FR main line in Blaenau Ffestiniog, see Maenofferen/Votty And Bowydd Slate Quarry Locomotive.