Maenofferen/Votty And Bowydd Slate Quarry Locomotive
Following the closure of the Festiniog Railway in 1946, the Maenofferen and Votty And Bowydd Slate Quarries at Blaenau Ffestiniog leased the 650 yards of line from Duffws to the LMS Exchange Station to transport slate and other freight until 1962-3. The annual rental of line and slate wagons, and associated wayleave charges, provided a continuing income to the railway. A siding was built by the quarry companies under the Queens Bridge to provide shelter for an internal-combustion locomotive that was used to haul trains.
One locomotive recorded in operation here in 1951 was a Muir Hill 4wPM owned by Maenofferen Quarry.
There is a picture in The Festiniog Railway, 130 Years in Pictures showing a Fordson Tractor-based loco which might be the Muir-Hill mentioned above. However, Boyd (1975 edition, vol 2 p488) mentions a tractor-conversion put together by Rhiwbach Quarry and acquired by Maenofferen being used on this duty and shedded at Queens Bridge. This might be the loco in the picture, which is similar but not identical to the Muir-Hill-Fordson acquired by the Ashover Light Railway in 1940. The layout is similar with exposed driving chains from the tractor rear axle to the leading axle of the underframe, but the frames and end castings are different. The Maenofferen Quarry page of this wiki states that the second-hand loco acquired by Maenofferen from Rhiwbach was actually a 1925 Muir-Hill, so it is probably the same one as in the picture. It was scrapped in 1964-5. There are several pictures on iBase, 1827, 1830, 1844.
There is an alternative explanation of the origin of this loco. According to Boyd's NG Railways in North Caernarvonshire, Vol 2 The Penrhyn Quarry Railways, this locomotive "may have been" constructed at Coed y Parc of the PQR from a Fordson Tractor off the Penrhyn Estate as Quarry No. 5 in 1934. It was sold to Pen-yr-Orsedd Quarry and then sold to and used by Maenofferen Quarry in the Ffestiniog Yards.
Another locomotive that may have worked over FR metals in Blaenau Ffestiniog was Hibberd 4wPM 1929 of 1935. This was noted lying off the track by the ex-GWR goods shed in 1954, and by then allegedly the property of Maenofferen Quarry.
Ruston & Hornsby
There is a picture in Boyd (1962 edition, vol 2 p347, not in later edition) showing a Ruston working slate from Duffws to the LMS yard in 1954. There are several pictures of the same or a similar Ruston in the book Festiniog in the Fifties working over FR metals around Queens Bridge, claiming this as an Oakeley Quarry loco. See also iBase 1751, 1752, 1795, 1797.
The Oakeley Slate Quarries built a direct line to the LMS Exchange yards in about 1946 which meant that they did not have to use the FR Co.'s closed Dinas branch. This also meant that they had direct access to the Duffws section. According to Boyd (1975, vol 2) Oakeley took over Votty & Bowydd in 1933 but the loco fleets do not seem to have been amalgamated. So it could have been an ex-Votty loco under Oakeley control, an actual Oakeley loco, or a Maenofferen loco with a mistaken ownership. All three concerns are listed as owning Rustons.
Hopefully this matter will be further clarified by later contributors.