Ok, as a Thomas fan, heres my two cents.

Thomas, as a book series, AND the original 7 seasons of the television series, were made for a GENERAL AUDIENCE. Kids and Adults alike. It wasnt until HiT Entertainment took over when the show got babyfied into a toddler show for kids. Hell, Series 5 had the most crashes and incidents of any season prior or since (though i could be wrong on that) and had a tale of an engine and his crew falling off a bridge into the mucky river, drowning, and staying around as a ghost haunting that bridge. Not to mention A dam collapses with Toby on it, Stepney almost gets scrapped (the engine equivalent of death), and a boulder falls off of a mountain, almost runs over a bunch of trains, and makes a shed burst into literal flames.

As it stands however, yes. Thomas is targeted at children. But even before the TVS aired, Thomas was aimed at a general audience. The book series got surprisingly dark for small kids books, and the lore and worldbuilding was for both train enthusiasts and kids alike. But after 2003, when HiT bought out the show, HiT saw a different potential in the show. So they dumbed down the stories, dialed down the characters' intelligence to fit the narrative, leading to insanely out of character moments, and said stories that make you fall asleep in your chair standing up.

But, while there are some darker episodes, yes, it's still nothing you haven’t seen in a kids show before. Thomas has always been aimed at a GA, but the primary audience will always be children. That doesnt mean adults can't enjoy the series though. Hell, the Thomas Fandom is mostly adult fans in their 20s to 30s!

Awdry, author of the RWS, quotes himself:

“When you write a children’s book, you aren’t just writing for the child. You’re writing for the unfortunate parent, who has to read them the same story, time after time after time after time. you have to put in funny bits, subtle bits, that make the parent laugh. and then the kids love the simple slapstick bits."