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Duncan is the eighteenth book in the My Thomas Story Library series.
Duncan moans about not being polished and refuses to take passengers. That night, Skarloey tells him a story about Rheneas who always put his passengers first. Duncan promises from then on to always put his passengers first.
- This book is based on the stories, "Passengers and Polish" and "Gallant Old Engine" from the Railway Series book, Gallant Old Engine.
- Peter Sam has his old funnel.
- In the fourth illustration and on the 2011 cover, Duncan is missing his number.
- Throughout the book, Duncan's cab handles disappear and reappear.
- Throughout the book, Duncan is missing his whistle.
- The viaduct is not to scale.