According to The Island of Sodor: Its People, History and Railways, the slate quarries on the Skarloey Railway were taken over by the Ministry of Defence after their closure for the storage of ammunition. The MoD uses their own engine to shunt the sidings in the quarry, a black, electric-powered engine described as "beetle-like". Due to the highly secretive nature of its work, the engine has never been seen.
The Ministry of Defence engine is based on the Simplex petrol locomotives built during World War I, hence its body armour. They were fitted with silencers on their cab roofs to prevent being seen by the enemy.
It is possible that the engine is based on the Armoured Simplex petrol diesel named Mary Ann, owned by the Ffestiniog Railway.