Duke, named after the Duke of Sodor and originally called The Duke, is a narrow gauge tank-tender engine, who is widely respected and considered a hero amongst all the engines. He is also one of the oldest engines on Sodor.

Duke is one of the oldest, wisest and most experienced engines of the Skarloey Railway. He is also the mentor of Peter Sam and Sir Handel, from their younger days on the Mid Sodor Railway, when they were called Stuart and Falcon respectively. Being the oldest engine on the line, Duke liked everything to be "just so". Although once lost for many years in an old shed that nature tucked away and forgotten about after the Mid Sodor came to an end, he was found by rescuers, although quite by accident when one dropped in through his shed roof. He may have a headteacher attitude and maintain his authority at all times, but can be quick to apply his knowledge and years of experience to help guide and correct any younger engine. He is proud of his name and keen to uphold the respect it merits. He can often be heard to say "That would never suit His Grace!" upon hearing or seeing something that he takes disapproval to, or pedantically stating how he thinks someone would have adhered to the rules and regulations in his younger days. Despite their teasing ways, Sir Handel and Peter Sam have grown to become very fond and deeply respect Duke and dearly nicknamed him "Granpuff" for his long service to the railway.