Luke is a plucky little pea-green tank engine on the Skarloey Railway, brought to Sodor from a neighbouring land. For a long time, he hid in fear and deep shame in the tunnels and caverns of the Blue Mountain Quarry from a terrible, dark secret, only in the knowledge of the other narrow gauge engines, believing that he had caused a Yellow Engine to fall into the sea. Fortunately, he befriended and trusted Thomas.