Marion is a self-propelled railway steam shovel, who usually works at the Sodor China Clay Company with Bill, Ben and Timothy. She is rather bubbly and eccentric, and sometimes comes off as being strange.
Marion first met Thomas when she was sent to clear away the fallen rocks from a landslide that had occurred the previous night, and came off to him as being rather talkative. She continued clearing away the fallen rock for a few days, when another landslide happened at the clay pits. During the landslide, Percy was nearly crushed by the skull of a dinosaur, but Marion managed to catch the skull in her bucket and identified the skull as being that of a Megalosaurus. She helped to dig out the rest of the skeleton, which was put on display at the Tidmouth Town Hall. (TVS; Tale of the Brave)
Marion was later given the task of repairing an embankment on the Main Line, which she did. However, as she was doing her work, Marion struck a water pipe, causing it to leak. Though she covered the leaky pipe back up, Thomas spotted water trickling out of the ground and questioned her about it. After failing to make up a good excuse, Marion went with Thomas back to the leaky pipe, which burst and caused a landslide. Marion quickly got to work clearing away the line, and managed to finish just as Gordon passed by with the express. (TVS; Marion and the Pipe)
After Ben failed to bring a train of coal back to the clay pits, Marion became concerned and ordered the twins to fetch it. However, Bill and Ben chose to waste their time goofing around instead, and ended up running out of coal. As he was the only engine at the clay pits who did not run on coal, Timothy went to Brendam Docks and fetched the train of coal, which he brought back to the clay pits. Marion then made the twins apologize to and thank Timothy. (TVS; No Steam Without Coal)
When a new shed was being built in a siding, Marion slept on the siding while it was being worked on, when she noticed Samson passing by with a train of animatronic dinosaurs. Marion did not see Samson, however, and believed that the dinosaurs were real. When she saw the dinosaur models for a second time, Marion ran away and raced all the way to Ulfstead Castle, where she discovered that the dinosaurs were not real. Sir Robert Norramby then gave Millie the job of looking for more dinosaur bones for his new dinosaur park, much to her delight. (TVS; Marion and the Dinosaurs)
When the Harwick Branch Line was under construction, the Fat Controller sent Marion to the construction site to help out. On her way to the construction site, Marion passed through a forest in Arlesburgh, when two small engines whooshed past her from behind the bushes. Marion became startled and shut her eyes, and ran into Oliver and Toad at the junction. Toad then introduced Marion to Rex, Mike and Bert, the three small engines. Marion was in awe at the engines' miniature size, and believed them to be magical. This belief was further fueled when she met Oliver the excavator, who she believed had turned himself into an excavator from a steam engine by magic.
Marion was present when Rocky was lifting the pirate ship out of the cavern, when she suddenly got the idea of making a wish to the small engines so that she could find pirate treasure. When she returned to Arlesburgh Junction, Rex and Bert both ran away, but she managed to trap Mike on the chute with her shovel. As Marion made her wish, Mike managed to escape and sprint away, but this only convinced her that they were magic. Her wish came true when she found an old chest inside the cliff face, which turned out to be Captain Calles' old treasure. When she attended the opening ceremony of the Harwick Branch Line, Marion discovered that there were, indeed, two different Oliver's. (TVS; Sodor's Legend of the Lost Treasure)