Thomas is a talking tank engine. He lives on an island called Sodor and basically drives around having minor adventures with his friends and moving passengers and freight.
Are the Thomas stories on-topic for any other reason than the trains talk (in which case, these seem to fall foul of our rules about talking animals), or can we identify anything else that would bring these back in line?
If we allow the basic conceit of sentient trains and other machines, the stories themselves are quite mundane. The same stories could be told with Thomas' role, and those of the other engines, replaced by their drivers, without it making much of a difference.
However, questions about the uplifting of the engines and the consequences of that, are on topic. Examples of this type of question can be found in this video by Tomska.
If fantastical elements beyond "in this universe vehicles and rolling stock are sapient and can talk" are required then I'd say no, the Railway Series would generally not be on topic. The one area I'd say you might consider it on topic is as to how the stories themselves affect the universe, if fourth wall breaking in that way can be considered fantastical.
The stories are mostly grounded in reality; in fact, many of them are factual events, transposed onto the Isle of Sodor. The engines are throughout treated as engines; the fact they can talk is not extraordinary in universe. This does sometimes bring up odd dissonances; people generally have no qualms scrapping engines, despite their sentience, just as people might have no qualms scrapping engines in our universe. (Though the books do claim it is "natural for an engine" to feel sad at the idea.)
Sodor itself of course does not exist in our regular universe; the series is set in a universe where there is a large island in between the Isle of Man and the British mainland. But other than this the history of the railway series follows our history exactly, and features visits from real life engines (such as Stepney and City of Truro) and people (such as Queen Elizabeth II).
One slightly fantastical thing is that the Railway Series itself exists within the world of the Railway Series. As the real life series became popular, the series also became just as popular within the world of the books; but, despite being in the same universe as the book series, the people of that universe also didn't believe the engines were real, just as the people in the real world don't.
"The people of England," he said, "read about Us in the Books; but they do not think we are real..." "Shame!" squeaked Percy. The Fat Controller glared. Percy subsided. "...so," he continued, "I am taking My Engines to England to show them."
The forewords by the author also generally keep up this melding of the alternate history with real world history, talking about how the engines might get conceited they have a book named after them, and similar.
There are tales about Ghost Trains, wizards, like in one magazine story where Boco tells a story to Skarloey of a wizard turning a fast, arrogant shiny engine named Silver into the slow and dirty Tramp, and everything about Thomas and the Magic Railroad!
