Duke is an eldery brown narrow gauge engine from The Island of Sodor .
Duke is one of the oldest and most experienced narrow gauge engines. He is also the mentor of Peter Sam and Sir Handel, from their younger days, when they were called Stuart and Falcon. 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 forgot, after his railway (Mid Sodor Railway) closed down, he was found by rescuers when one dropped in through his shed roof. In spite of his head teacher attitude, he can be quick to apply his knowledge and years of experience to help guide and correct any younger engine. He is proud to be named after The Duke of Sodor and he is keen to uphold the same respect. He can often be heard to say "That would never suit His Grace!" upon hearing or seeing something that he takes a disapproval to, or state that things would have been different in his younger days. Despite their teasing, Sir Handel and Peter Sam are very fond of Duke and dearly nicknamed him "Granpuff".
Duke is based on an engine called Prince, a George England tank engine from the Ffestiniog Railway at Porthmadog, North Wales. Unlike his basis, Duke is built to 2ft 3in gauge, while Prince is built to 1ft 11½in gauge. Bertram is another member of this class.
Duke is painted brown with yellow lining and has two red plaques on either side of his saddle tank with his name written in gold.