Before coming to Sodor, Rex used to work on an unnamed miniature railway in England with Bert and Mike. When their railway closed, the Fat Controller brought them to Sodor to work on the Arlesdale Railway. Rex later befriended and introduced Bert and Mike to Duck.
When the railway managed to secure a deal to transport wool bales, Rex was given the task of taking the train due to his excellent knowledge on the subject. However, when a negligent farm-hand spilled a tractor's load of wool bales onto the line, Rex ran into them and derailed. Though his train was rescued and delivered, Rex had to wait several days until he was re-railed. (RWS; Small Railway Engines)
In 1976, Rex had a badly leaking steampipe and had failed while pulling a passenger train. Frank was sent to rescue Rex's train. After the event, the Small Controller began thinking they needed another engine and so he arranged for Jock to be built. (RWS; Jock the New Engine)
During the construction of the Harwick Branch Line, Rex, along with Mike and Bert, were put in charge of bringing ballast trains down to Arlesburgh Junction for the standard gauge engines to take to the construction site. When Marion met the small engines for the first time, Rex, along with Bert, treated Marion kindly, whereas Mike was less than kind. When Thomas was feeling depressed, Rex and the other two engines sang "Never Overlook a Little Engine" to cheer him up, but this only re-aroused his overconfidence.
Later that evening, Rex was pulling an empty ballast trucks alongside Bert, when Marion arrived at Arlesburgh Junction. After teasing Mike, saying that she is looking for him, Rex quickly left with Bert, leaving Mike with Marion.
Two nights later, as Sailor John was passing through the junction with the pirate ship, Rex was the first one to recognise Thomas' whistle. He quickly thought of a plan to try and slow down the pirate ship, and got Mike and Bert to help him push loaded ballast trucks off the chute, and onto the tracks. As Thomas raced through the junction, the three engines encouraged him to keep chasing Sailor John. (TVS; Sodor's Legend of the Lost Treasure)