Sidney is a forgetful diesel shunter.
Sidney was one of the diesels who laughed at Percy's joke when the tank engine first arrived at the Sodor Dieselworks. He only appeared very briefly on a hoist with no wheels.
He was still there in the seventeenth season and was often referred to as a Christmas decoration by the diesels, literally being used as one by Diesel 10 as well. Percy asked how long Sidney had been left up on the hoist and found out from Norman that it had been two years. Percy gave Sidney a new set of wheels as a Christmas present, and Sidney is now a really useful engine again.