Rex is a small green tender 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 a negligent farmhand spilt 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 is based on the Ravenglass and Eskdale Railway's River Esk.
Rex is painted green with red and yellow lining, a brass dome, red nameplates with gold writing mounted each side of his smokebox and "Arlesdale" written in gold on his tender.