Marion works at the China Clay Works. She is bubbly and friendly and loves digging, talking, and talking about digging. She is always ready to share her tales with the other locomotives, who sometimes try to avoid her. Thomas is always happy to hear her stories. Items she has claimed to dig up in the past include a pirate chest. She enjoys playing "Guess What's in My Shovel", although the answer is almost always mud, rock, or dirt.
The morning after a large landslide at the clay pits, Marion met Thomas for the first time, scaring him with her bucket. When she introduced herself, Thomas found that she talked a lot. A few days later, she was still cleaning up the fallen rocks when Percy and James came her way trying to escape from another landslide caused by James' whistle. Although James escaped, Percy was swept up by the landslide. He was buried wheel-deep when he saw a large rock coming his way. It looked like the end for the little green tank engine, but Marion put her bucket in front of him and caught the rock, which she identified as the skull of a Megalosaurus.
Whilst repairing an embankment on the main line, Marion once hit a water pipe, causing a leak. Marion decided to cover up the leaky pipe, but Thomas later spotted water trickling out of the ground. When he took Marion to inspect the site, water gushed up from the ground and caused a landslide. Luckily, Marion was able to clear the line just before Gordon raced through with the Express. Marion promised that she would tell someone if she unearthed a water pipe in the future.
Later, Marion became worried when Ben did not bring any coal to the clay pits. She was cross when she realised that the twins had used it all up, and got them to thank and apologise to Timothy, who came to the rescue by fetching more coal for Marion and the twins.
Some time later, during the construction of the Dinosaur Park, Marion was sleeping on a siding when Samson pulled a train of animatronic dinosaurs past her. She did not see Samson and believed the dinosaurs were real. She reversed all the way to Ulfstead Castle, and a few minutes later Samson arrived, revealing that the dinosaurs were in fact fake.
In the nineteenth season, Marion witnessed Bill and Ben taking away trucks she was loading, causing rubble to land on the tracks. She says that they aren't helpful, and Timothy agrees. This infuriates the twins, and as a trick, send Timothy to the Docks to help Porter and Cranky. This only caused trouble at the Clay Pits because Marion's trucks were getting too full. She ended up getting trapped due to the heavy trucks. Bill and Ben framed Timothy, angering her. Despite Timothy coming back, Marion was still trapped for a while.
She was brought in to help with the construction of the new branch line. Upon meeting Bert, Rex and Mike, she believed that they were magical - a thought that became more remarkable when she thought they turned Oliver into an excavator. Marion was present when Rocky lifted The Pirate Ship out of Calles Cavern, and decided to make a wish to the small railway engines that she would be the one to dig up the buried treasure. The three of them were less than enthusiastic when the steam shovel arrived at Arlesburgh West, and while Rex and Bert managed to flee, Mike got trapped on the chute by Marion's shovel, and she did not let go until she had made her wish. The little red engine escaped in full speed, but this only strengthened Marion's belief that he was magical. She helped Alfie and Oliver dig part of the site when she discovered a buried treasure chest. She later attended the branch line's opening ceremony, and was surprised to discover that there are two Olivers on Sodor.
In the twentieth season, Marion was working on Thomas' Branch Line to help make room for a new warehouse on a disused siding and discovered a "talking Christmas tree". Marion tries to tell Thomas, Toby and their coaches about her discovery, but they do not believe her. She then went to Knapford to get Sir Topham Hatt to take him to the talking Christmas tree, but when they arrived it was gone. Marion was later recognized for Glynn's discovery.
Marion is very enthusiastic and likes to think of herself as well-informed and is happy to share her knowledge. She is very talkative and loves to tell stories of her discoveries. However, not all the engines prefer to stick around and hear her stories.
Despite her enthusiastic demeanour, she can also be rather stern, as seen when she told off Ben for not delivering any coal and making the twins apologise and thank Timothy.
Marion is based on a rail mounted Marion Model 40 Revolving Steam Shovel. However, unlike Marion, the actual steam shovels were not self-propelled. Many of these steam shovel types were used in the construction of the Panama Canal, with some of them still in place at the bottom of portions of the canals and reservoir that provide water to the canal and locks.
The only remaining steam shovel is a Model 91, which was taken out of service in 1949. It resides in Le Roy, New York, USA on caterpillar treads. It is on treads because in 1916, the Marion Company began selling conversion kits, to allow the shovel to be placed on caterpillar treads. Model 91 is listed on the National Register of Historic Places.
Marion is painted orange and light brown with yellow lining. She has her name painted on her sides in yellow.