Thomas is a tank engine who works on Thomas' Branch Line. He came to Sodor in May 1915 as a station pilot at Vicarstown until 1946, at which time he was transferred to Wellsworth following an incident with some trucks. Later that year, after rescuing James from his crash, he was given charge of the Knapford-Ffarquhar branch line and two coaches, whom he named Annie and Clarabel.
Thomas started his life on the Island of Sodor as a cheeky little engine who shunted coaches for the Main Line engines, such as Henry and Gordon. He liked to play tricks on them, including Gordon, "the biggest and proudest engine of all". He was taught a lesson when Gordon tricked him into being coupled to him during his express run. Tired out, he vowed never to tease Gordon again, realising he did not have to be important. Soon he desired to pull his own passenger train too, much to the bemusement of the other engines. He got his chance when Henry became ill, and no one could pull the passenger train. However, Thomas was so excited that he accidentally left his train and his passengers behind, believing he was pulling it easily. He soon realised his mistake and returned to pull the train. The engines laughed at him, though, and soon Thomas wanted to get out of the shunting yards and see the world. Edward offered him a chance to pull trucks, and Thomas excitedly took the job, not realising how troublesome trucks could be. The trucks pushed him down Gordon's Hill into a siding, where the Fat Controller told him to practice so that one day he would become as good as Edward. Thomas soon proved himself when he rescued James after his accident with the trucks, using The Breakdown Train. As a reward for his quick and heroic action, the Fat Controller promised him his own branch line.
Thrilled, Thomas soon began running his branch line with his two new coaches, Annie and Clarabel, who he grew to love very much. He soon considered his branch line to be the most important part of the entire railway, a fact he would constantly remind the other engines of. However, he continued to make mistakes and dream big, and sometimes this got him into trouble, such as when he was so impatient that he left his guard behind at Elsbridge or when he wanted to fish, not realising the implications of a fish being in his boiler. His greatest mistake was when he got his snowplough broken in the winter and ended up stuck in a snow drift, having to be rescued by Terence, whom he had insulted earlier for not having "real" wheels. He soon proved himself again, though, when he beat Bertie the bus in a great race. He quickly became friends with both Terence and Bertie, despite their differences.
Although Thomas' branch line was indeed the pride of the line, on the main line, things were not going well for the Fat Controller, who was having to deal with Gordon, Henry, and James' sulking, refusing to do Thomas' former work of shunting themselves. After they went on strike, Thomas was brought back to help alongside Edward and their new friend, Percy, who quickly became Thomas' closest friend. Thomas also became friends with Toby after he was asked by the Fat Controller to help their railway avoid a lawsuit from a rude policeman. But Thomas still had some cheekiness in him, returning to teasing Gordon after Gordon slid into a ditch. He soon needed Gordon's help after he blatantly went past a danger sign and fell down a mine. They soon became friends after that and promised never to tease each other again. Thomas and Toby later helped Mrs. Kyndley at her cottage after she was snowed in and helped her celebrate Christmas with a very special Christmas party.
The following year, Thomas shared a shed on his branch line with Percy and Toby. However, he became conceited about his brilliant blue paint and soon paid the price when Percy accidentally caused a coal hopper to pour coal all over him. Thomas was angry at Percy, but the two friends soon made up after Percy had an accident with some coal trucks. Thomas soon met Trevor, a friend of Edward's, who proved his usefulness when he helped Thomas at the newly constructed harbor. Unfortunately, Thomas became ill a few months later and had to be sent to The Works, while Duck looked after his branch line. When Thomas came back, his brakes were stiff, and made it seem as if they were hard on. As a result, one day, when his fireman was ill, a substitute accidentally caused Thomas to run away, and Harold had to help stop him. Later, when The Viaduct needed to be repaired, the main line engines became late to drop off Thomas' passengers and, as a result, made him late for Bertie. Thomas continued to be of great service to Sir Topham Hatt's railway, although he still remained a cheeky little engine.
Thomas later became conceited again when he took his driver's compliment to heart and believed he no longer needed him to work his branch line. This was untrue, but Thomas was too puffed up with himself to not believe it and, one morning, found himself rolling along the line, when a cleaner had accidentally started him up. He soon broke into a stationmaster's house and ruined the family's breakfast. He had to be rescued by Donald and Douglas. As a result, the Fat Controller angrily sent him to the works again. He arrived back a few weeks later just in time to take over for a battered Percy, who just had a predicament with some trucks. The following Christmas, Thomas was eager to sing carols with the other engines, but had to pick up a Christmas tree for the Fat Controller. Unfortunately, on his way back, he was caught in a snow drift and was lost. Donald and Douglas soon rescued him in time to sing carols with all his friends.