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Rex is a green engine who enjoys nothing more than teasing Mike. He is a hard worker, but sometimes has problems with traction.
After Rex's old line in England closed down in 1967, he, Mike, and Bert were brought by Sir Topham Hatt and transferred to the Arlesdale Railway for mining and tourist traffic. After the railway secured a deal to transport wool to Arlesburgh, Rex was chosen to pull the first train. But after Willie, a negligent farmhand spilled some wool bales on the rails, Rex hit them and derailed. His train was rescued, but Rex had to remain there for several days until he could be lifted out.
Rex later broke down when his steampipe was leaking, but when his driver used his radio telephone to contact for help, Frank was available, and took his train while Rex backed into a loop and went back home for his driver to mend his steampipe.
When Marion was heading to the construction yard for the Harwick Branch Line, she was startled by Rex, Bert and Mike at Arlesburgh Junction. Rex told her that their railway catered to passengers, as well as ballast and wool. Marion had never seen such small engines, and thought that they must be magic. Later, Thomas arrived at the Junction, dismayed that The Fat Controller had given his branchline to Ryan. He, Mike and Bert sang Never Overlook A Little Engine to lift his spirits. Later on, when Marion arrived at the Junction again and addressed the three engines as "magic engines," Rex joked that she was talking about Mike, and he and Bert left before Marion could notice them, leaving Mike. Two nights later, Thomas was chasing Sailor John, Skiff and The Pirate Ship. When they reached the junction, the small engines were woken up by Ryan, who believed he saw pirates, and quickly thinking, they shunted a train of hoppers off the chute and onto the standard gauge line to slow them down. The ship crushed most of the hoppers immediately, but one was caught under it, causing it to lose speed. Ryan then managed to derail the ship.
Rex is a playful green miniature engine with a strong sense of humour. He is a hard worker, and is the natural leader of the little railway engines (in spite of the fact that Mike prefers to think he’s the leader). Rex is very proud of his railway and a great friend to the other engines, but he can’t help teasing Mike to stop him getting too full of himself. Most of the time Mike manages to return the banter, but sometimes he gets a little wound up. Luckily Rex is very good at helping Mike see the funny side again.
Rex is painted green with red and yellow lining. He has a brass dome, brass builder's plates on the sides of his cab, red nameplates with gold writing mounted on each side of his boiler and "ARLESDALE" written on the sides of his tender in yellow. In the Railway Series his wheels are painted black, but in the television series they are green.