Sidney is a forgetful blue diesel, belonging to the Fat Controller.
Sidney was first seen at the Sodor Dieselworks having his wheels replaced and when he was repaired, the Fat Controller gave him a very important special Special Sidney was to go to the Maze in the woods and try it out and try not to get lost and Sidney replied with a toot of his horn and he set off.
In Day of the Diesels, Sidney asked Thomas if he would like to visit the Sodor Dieselworks instead of delievering the Water Tanker for Belle the new Sodor Fire Engine to the Sodor Search and Rescue Centre but Thomas he shouldn't go to the Dieselworks but he accepted and Sidney thanked him but he saw Percy with Flynn the Fire Engine and Sidney hurried off.
- Kerry Shale (UK/US Day of the Diesels only)
- Bob Golding (UK/US seventeenth series onwards)
- Sidney is painted blue with yellow lining and a warning panel underneath his face.