The railway first thought of using a diesel locomotive in the early 1970s, when a small four-wheeled diesel-mechanical locomotive built by Ruston & Hornsby (their class 48DL) was bought second-hand from a quarry. It was intended to regauge it and use it as a yard shunter at Llanberis. It was sold to the Llanberis Lake Railway in 1978 without being regauged or used on the SMR. It would have been the railway's only locomotive without pinions, and as such would have been of limited use – it is doubtful if it would have had sufficient grip on the grease covered rails to shunt a dead steam locomotive. This locomotive has since been dismantled and scrapped.