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.
Rex is based on a design by Mr. Henry Greenly, and was built by Davey, Paxman of Colchester in 1923 at the same time as River Esk was for the Ravenglass and Eskdale. He was found to be powerful but erratic and was sent to the Yorkshire Engine Co. for rebuilding in 1928 at the same time as River Esk, and for the same reasons. Walschaerts valve gear, substituted for the poppet gear originally fitted, gave beneficial results.
After Rex's old line in England closed down in 1967, he, Mike, and Bert were bought by Sir Topham Hatt and transferred to the Arlesdale Railway for mining and tourist traffic. Since coming to Arlesburgh Rex has been found reliable and free steaming, but needs careful handling with any but the lightest loads, having an unfortunate tendency to slip. 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 West. 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 sent him to work on the new branch line, while Ryan looked after his own. 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 reprises his roles from the Railway Series, from when he and Bert teased Mike about his whistle to when he derailed from crashing into the wool bales that fell on the tracks from Willie.
In the twenty-first season, instead of bringing presents to the children this year, Rex brought a truck of fake snow for the fan at the end of his train to blow on it, making it look like it was snowing. Daisy, who was present when it happened, was not happy about it since it wasn't the way she wanted it.
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.
- Small Railway Engines - Ballast, Tit for Tat, Mike's Whistle, and Useful Railway
- Oliver the Western Engine - Donald's Duck (not named; mentioned) and Bulgy (cameo)
- Duke the Lost Engine - Sleeping Beauty
- Jock the New Engine - We Need Another Engine, Sticking Power, Jock, and Teamwork (cameo)
- Wilbert the Forest Engine - Wired-Up (mentioned)
- Season 20 - The Way She Does It (cameo), Tit for Tat, Mike's Whistle, and Useful Railway
- Season 21 - Daisy's Perfect Christmas
- 2015 - Sodor's Legend of the Lost Treasure
- 2016 - The Great Race
- 2017 - Journey Beyond Sodor (cameo)
- 2007 - Thomas' Giant Coloring Book
- 2008 - Thomas' Sticker Express
- 2015 - Pirate Treasure, Sodor's Legend of the Lost Treasure (does not speak), Sodor's Legend of the Lost Treasure: Sticker Activity Book, and Sodor's Legend of the Lost Treasure: The Movie Storybook
- 1985 - Oliver Gets the Bird (mentioned)
- Tom Stourton (UK/US)
- Yuta Odagaki (Japan)
- Przemysław Stippa (Poland)
- Stan Limburg (The Netherlands)
- Óscar Flores (Latin America; Sodor's Legend of the Lost Treasure only)
- Gerardo Mendoza (Latin America; twentieth season onwards)
- Bernd Lambrecht (Germany)
- Yannick Blivet (France and French speaking Canada)
- Dan Schatzberg (Israel)
- Rex uses Mac's original whistle sound.
- In Rex's trading card promo, he is depicted as a 4-8-2 along with Mike and Bert.
- "What do you think of our 'chute?" said a voice. "Good, isn't it?"
Duck blinked. Standing beside him was a small green engine.
"Where did you spring from?" asked Duck.
"I've been here all the time," smiled the small engine. "I'm Rex, and you, I'm sure, are Duck."
"How did you know?"
"That's easy; there's only one Great Western engine in these parts."
- ―Duck meeting Rex, Small Railway Engines