Sidney, nicknamed "Sid", is a bit of a forgetful engine. He starts his day at one location and ends up at another, not remembering how he got there. When it was his turn for repairs at the Dieselworks, he was hoisted up and had his undercarriage removed. Being patient, Sidney was happy to wait, and wait, and wait. He sat there happy to pass the time with any engine that happened to be visiting. Two years later, he still had no wheels, so Percy helped him at Christmas to get a new set of wheels and now he is back on track.
- The Lorries (cameo)
- Catch of the Day (cameo)
- The Christmas Catastrophe (cameo)
- Sidney's horn is actually Paxton's at a different pitch.
- Despite Sidney's nickname being "Sid", he has been consistently referred to by his full name rather than his nickname. He shares this dilemma with Paxton.