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Sidney is a small saddle tank engine who runs a narrow gauge branch line near Duck's branch line.
When Sidney first arrived on Sodor, Duck had to collect the new engine from the Docks and deliver him to a Scenic Railway on his goods train. Sidney started to boast about the view that his railway had to offer, and Duck argued with him saying that his branch line was more picturesque. When he managed to show Sidney all the splendid sites that Sodor had to offer, Sidney even admitted that maybe he was wrong about whose line would have better views.
Duck promised Sidney that he would bring plenty of passengers to him on his new railway at the Holiday Camp. Later, Sidney was relocated to another narrow gauge branch line near the seaside. He met up with Duck again, and they've been firm friends ever since.
Sidney can be rather boastful, single minded, and above himself. However, this attitude changed when he befriended Duck. Sidney enjoys chuffing around the seaside, pulling passengers to the funfair and back.
Sidney appears to be of a freelance design.
Sidney is painted baby blue with yellow lining. He also has a red and yellow nameplate with black lettering on either side of his tanks. His dome and safety valves are also painted yellow, and his wheels are painted black. He has gold on his funnel.