Rex is a green tender engine who works and lies on the Arlesdale Railway.
After Rex's old line in England closed down in 1967, he, Mike, and Bert were brought by The Fat Controller 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 has a friendly, often argumentative rivalry with Mike.