This ‘thirteenth’ train was built for use on the 40cm gauge line of the Romney, Hythe and Dymchurch Railway (RH&DR), once this line had been requisitioned by the army on 26 July 1940. Work had already begun on 16 July on converting the locomotive originally chosen, a diesel shunter, but it became immediately obvious that the additional weight to be carried could not be supported by the bogie and the two driven axles. A steam engine was therefore selected for conversion, the famous 4-8-2 Hercules of the RH&DR. The work of fitting the armour took one month, and protected the whole engine except for the driving cab, which was left open at the rear. The two armoured wagons, former mineral wagons, were coupled in front of and behind the engine. Each wagon was armed with a Boys anti-tank rifle and a coaxial Lewis LMG mounted behind a shield on a ring at one end, with a second Lewis on an anti-aircraft mounting in the central compartment, but without a shield.
For all its reduced dimensions, with its four Lewis Guns the Miniature Armoured Train was credited with one victory, a Heinkel III, and one probable, a Messerschmitt Bf 109 on 7 October 1940.