Grandfather Hatt is the grandfather to Sir Topham Hatt (who was known as "Little Topham Hatt") and father-in-law to Dowager Hatt. He is presumably the original manager of the North Western Railway or one of its predecessors.
One day, both he and his grandson were out on a carriage ride through the countryside, but Grandfather Hatt noticed a hole on the road and the carriage's wheel broke off, leaving the horse running away in the distance.
They later passed Stepney who was pulling coal trucks. and despite the load, decided to hitch a ride on the trucks back home. They were covered in coal dust, but they didn't mind it, and later took their photograph.
Similar to his grandson, Grandfather Hatt is a short, stout gentleman. He is dressed in a black morning suit with grey trousers, a red waistcoat, a black tie and a top hat. He is also seen with a white moustache.
- Thomas-saurus Rex (does not speak)
- The book never stated if he was the original Sir Topham Hatt like in the Railway Series.