I'm just going to briefly summarise this show:
Series 1-4 are great, more or less directly based on the Railway Series books written by the Rev. Awdry from 1943 to 1972. These "children's" books were written with the intention of being as realistic as possible (minus the engines having faces), hence why nearly every singe story is based on an actual event, and most importantly; they were not just written for children. They never played down to children whatsoever, and neither the first four series of the TV-adaptation.
Series 5 was released after Awdry's death and featured stories not based on the books. However, many fans consider it to be the best series of the show with its much darker and more mature tone and it's many crashes and explosions...
After series 5 came the first film "Thomas and the Magic Railroad". Long story short; the film had nothing to do with the show, all the realism was replaced with magic and if you are under the assumption that Thomas is just a baby show, you are because of this film.
Series 6 and 7 came after the film and were a transition to a new era: Rather than the railway, morals and life lessons became more of a focus.
During series 7, the show was sold to HiT Entertainment and they finally made it a pure children's show.
The continuity goes: Series 8 -> Calling All Engines! -> Series 9 -> Series 10 -> Series 11 -> The Great Discovery -> Series 12.
During Series 12, they started utilising more CGI-elements and finally, the entire show switched to CGI, starting with the film "Hero of the Rails" in 2009.
The "Nitrogen Era" (animated by Nitrogen Studios) had awful episodes, but a few good films. Continuity:
Hero of the Rails -> Series 13 -> Misty Island Rescue -> Series 14 -> Day of the Diesels -> Series 15 -> Blue Mountain Mystery -> Series 16.
Blue Mountain Mystery was another start of a new era as HiT Entertainment was bought by Mattel. They got a new team for Thomas, that tried to make the show good again. Led by head writer Andrew Brenner, the "Brenner-era" tried very hard to use the original source material as a basis for a show of substance. And they more or less succeeded.
King of the Railway -> Series 17 -> Tale of the Brave -> Series 18 -> The Adventure Begins -> Series 19 -> Sodor's Legend of the Lost Treasure -> Series 20 -> The Great Race -> Journey Beyond Sodor -> Series 21
(Some continuity is a bit unclear as the production- and release history was messy).
But after that amazing run, Mattel changed the show again: Now called "Big World! Big Adventures!", it was now about Thomas travelling the world and the animation became much more cartoony than realistic.
Big World! Big Adventures! The Movie -> Series 22 -> Series 23 -> Series 24.
And after that failed, the show ended in 2021 to be replaced immediately by a 2D-Cartoon called "Thomas & Friends: All Engines Go!". The first episodes are currently airing and it is the worst thing that has ever happened to the show.
But the legacy lives on as there are many dedicated fans who are building their own models, animating their own engines or write their own stories. There are reasons why this "Kids"-show is still so beloved by teenagers and adults.
So, that was my first "comment" on this site.
("Edited" because I accidentally hit enter too soon)
