The oldest of the railway’s diesel locomotives is Chilmark, a 40DL diesel-mechanical 3-speed, which was built in 1939 for the Air Ministry. She was used by the RAF for hauling explosives and ammunition at RAF Chilmark in Wiltshire.
Chilmark was purchased by the railway in December 1976, and was extensively rebuilt soon after her arrival at Llanuwchllyn. A mechanical problem kept her out of use for some years. However, following her purchase by Patrick Ridley-Martin in 2013, she has been fitted with a replacement engine, together with several upgrades and improvements, and has now returned to the operational fleet of locomotives.