Forger Sherlock, forget Dr. Who, and forget Supernatrual.
Thomas the Tank Engine is one of the most well thought out and complex protagonists in a children’s series.
Starting off as a meek, excitable, and childish engine, he loved playing jokes on the engines, but he wasn’t particularly well liked by them, except Edward, who could be said to be a big brother figure to him. It became obvious that his constant jokes were cries for attention, as he was tired of shunting in the yard and wanted to explore the world. He wasn’t happy with the mundane, he wanted to be special. However, the first few tastes he had of the “real world” were not pleasant (tricked into running with Gordon’s train, forgetting his coaches, being pushed by freight cars). Despite this, he was determined as ever to go out on his own. Recognizing this, he was given his own branch line as a reward for saving James.
However, he clearly overestimated himself. He’d leave his conductor behind, beat up his snowplow because it was uncomfortable, and in general was a kid being given a big shiny toy. Despite this, he was convinced he was the most important part of the railway. He eventually meets Percy, who takes his old job as the shunter in the yard. The two become fast friends and Thomas acts as a mentor to him as well, however questionable of an influence he may be. Even though he was running a branch line, he was still irresponsible and goofy, teasing Gordon about falling into a ditch until he ran into a mine. Despite this, he would still tease Percy, fancying himself to be above him. The two bickered a lot, from coal to ghosts to wooly bears, possibly stunting Thomas’ growth. For most of the second season Thomas is out of focus, though what we do see of him he seems rather arrogant until he has his accident with the stationmaster’s house.
It’s obvious to tell from here that he has what could be considered a heroic BSOD. He isn’t the invincible engine he cracked himself up to be, and to add insult to injury, he’s temporarily replaced with a diesel. The idea of the steam engines being replaced hangs over them for much of the series, and even temporarily, it took a toll on Thomas’ psyche. He comes back a wiser, smarter engine, and after one more childish quarrel between him and Percy, he matures into a role model to the little green engine.
By season 3 he takes an extremely different tone. While he does make occasional quips, he mentors Percy much like Edward did the same to him and almost always works with a smile on his face. We see his anxiousness of being replaced again reappear in Thomas Gets Bumped, where he puts his job on the line to get some children home. However, the Fat Controller recognizes his big heart, and lets him off for it. Similarly, whereas he used to think himself as the best, he clearly takes offense when James insists the same, and says “we’re all useful, the Fat Controller says so, and he’s the head of the whole railway.” None of this is any more prominent than Trust Thomas, where he works his hardest to make all of his friends happy, and even when he gets hurt, he still wears a big smile on his face. His replacement complex is evident in Thomas And Stepney, where Thomas is jealous of the attention Stepney gets, along with the fact that he was shunted to let him pass. Despite this, Thomas is a sympathetic individual and the two become good friends.
Thomas begins the series with most everybody looking down on him, and through steam, coal and tears, becomes one of the most respectable and admirable characters on the whole railway, who pours his heart and soul to make sure he does a good job and everyone is happy. While the series deconstructs the troubles that come along with wanting to be “special” it also shows that it is also possible to overcome these troubles.
tl;dr: Thomas starts off a brat, wants to be special, realizes being special is hard, and grows up.
…and let’s forget anything after season 5 happened.