According to The Island of Sodor: Its People, History and Railways, the Slate Quarry near Skarloey was taken over by the Ministry of Defence for the storage of ammunition after its closure. The MoD uses their own engine to shunt in the sidings, a black electric-powered engine described as "beetle-like." Due to the highly secretive nature of its work, the engine has never been seen, and its basis is unknown.
“Oh dear this page is in a terrible muddle. It needs to go to the works to be mended. Can you be a really useful engine and help?”