Actor Allusion: Lady is voiced by Britt herself, and also shares the same color scheme as the Queen of Night from Mumfie, another show Allcroft worked on before.
Blooper: In the small arc about Donald and Douglas, their names were painted on them at the end because of confusion...however they seemed to film some scenes out of order, or flubbed the models used because you can spot them with their name tags on.
Someone with a keen enough eye will notice switchboxes or props in the scene. oops.
Sometimes engine's faces will placed improperly or have the wrong facial expression (in "Thomas And Trevor" for example, Thomas' facial expression is one same happy face model throughout the entire episode, no matter what emotion the narration claims he expresses).
Despite most of the fandom praising its animation quality, Arc Productions makes many silly errors in their first few works: points being set the wrong way, Oliver and Duck being out of proportion with their surroundings, brake vans changing on a whim, and so on and so forth.
Creator Backlash/Creative Differences: While the Awdrys often cooperated with the show on early seasons, and even wrote some new novels for extra material, Rev Awdry was rather critical of its first attempts at original stories due to their lack of realism towards railways, most infamously "Henry's Forest", as well as "crane shunting" Thomas into unlikely stories to keep him the main character. Christopher Awdry also disliked some of his newer stories to contribute to the show, as well as being forced to make more Thomas-centric stories to tie in on it's main character. Allegedly the Awdrys' ties with the show were more or less severed after the reverend wrote angry letters to Mitton and Alcroft about the state of the third and fourth seasons, considering them to be acting "pompous" with his work.
To a lesser extent original director David Mitton, who was cynical about the CGI transition, but otherwise had faith in the series going on after his departure.
Hit story executive Sam Barlow admitted that the constant rhyming theme of the CGI seasons (especially Season 15) probably wasn't a great idea.
Executive Meddling: Thomas and the Magic Railroad got butchered thanks to Hollywood executives being ignorami.
Specifically, the film was not received well by test-screen audiences, stating that the plot and main villain were too dark, despite the film being complete at that point. The end result was a mess riddled with plot holes that resulted in HIT Entertainment taking control when the film failed. Later, when HIT was planning on bringing Thomas back to the television screens in America, they chose PBS, whose educational stance forced the show to be retooled extensively.
The Shake Shake Bridge from Misty Island Rescue was originally supposed to be a girder bridge, which would sway when a character crossed it but was otherwise durable and safe. The animation team, however, were instructed to animate a wobbly old bridge that was falling to bits, which resulted in the intended moral being butchered.
Henry was twice planned to be written out of the series. In his initial story he was intended to remain bricked up in the tunnel and remain disconnected from Sodor, however publishers requested a happy ending for Henry that has him interact with the other engines of the book. Later, Awdry was frustrated with Henry's similarities to Gordon, so intended to silently 'kill him off' due to his failing health, though fans and publishers objected and so Awdry instead gave him an Emergency Transformation to establish a new distinctive design for the character.
God Never Said That: One major rumor surrounding P.T. Boomer before his full role was discovered was that he was planning on destroying Shining Time Station and replace it with a highway. An interview with Doug Lennox, the actor who played Boomer, said that though such a plan sounded quite villainous, was never in any version of the script to begin with.
Keep Circulating the Tapes: The 1999 Fox Family series Storytime With Thomas. It would feature two episodes from series 1-5, sandwiched with an episode of Britt Allcroft's other show The Magic Adventures Of Mumfie in between. No VHS or DVDs were ever released of the show, and it was never sold to other networks.
Lost Episode: Though 'The Missing Coach' was never finished, the production team took a few still shots corresponding to this story. When these got out on the Internet, for a while some people did think the entire thing had been filmed.
Name's the Same: In the UK, Season 2 and Season 11 both had unrelated episodes titled "Dirty Work," but in the US, the former, taken from the Railway Series' Duck and The Diesel Engine, was re-titled "Diesel's Devious Deed." The US is not immune to this, however, as it had a Season 1 episode called "Thomas Saves the Day" (its British title was "Thomas and the Breakdown Train") and a Season 8 episode also called "Thomas Saves the Day."
The UK did this again with season 1 and season 11 where both episodes "Thomas in Trouble" (the US version of said season 1 episode is re-titled "Thomas Breaks the Rules". Also the season 4 and season 8 episodes name is entitled "Fish", which the name stays the same in all UK and US versions.
The Other Darrin: Rupert Degas, who originally voiced Bertie, Butch, Dart and Flynn, departed following the production of Series 16. His roles were given to Keith Wickham, Matt Wilkinson, Steven Kynman and Ben Small respectively, who replicated the characters' original voices.
In Japan, the entire show was recast after the eighth season when the show switched stations.
The Other Marty: Various voice actors in Thomas & the Magic Railroad were replaced during the post-production overhaul:
Thomas was initially voiced by John Bellis, a cab driver the crew met while shooting on the Isle of Man. He can be heard in one of the teasers.
The series' concurrent UK narrator, Michael Angelis, was the voice of Percy and James. Test audiences thought the characters sounded too old, and in the final version they're both Cross-Dressing Voices.
A few of the trailers feature Diesel 10's original voice actor, Keith Scott.
Rupert Degas was supposed to voice Diesel 10 in Misty Island Rescue, but he was replaced with Matt Wilkinson after his lines had been recorded.
After Michael Brandon's departure, Martin Sherman took over the role of Diesel in the US.
The Pete Best: George Carlin joked about this after he replaced Ringo Starr.
Playing Against Type: George Carlin, the legendary comedian famous for his profane material played Mr. Conductor in the US version. Carlin stated in an interview he had taken the role with the direct intent of doing this and showing a different side to him.
Reality Subtext: Diese's basis, the Class 08, was a diesel responsible for displacing countless steam engines, especially tank engines. Further emphasizing this is just how many Class 08s Sodor currently has.
Recursive Adaptation: The show garnered it's own magazine series, with some of it's original stories actually adapted into episodes of the show itself in Seasons Three and Five. Incidentally the magazine's writer at the time eventually ended up lead writer for the show come Season Seventeen.
Ironically, this was also one of the reasons the original team stopped adapting stories from The Railway Series, feeling they had already adapted most of the unique stories their budget could handle.
The plot of Season 17's "No Snow for Thomas" is remarkably similar to Season 8's "Thomas, Emily and the Snowplough". Arguably a case of Tropes Are Not Bad, since reviewers who noted the similarities also commented that the former episode's plot is basically the latter's, but better.
Similarly "Duck In The Water" uses a lot of similar cues as "James To The Rescue", but is considered one of the best episodes of the CGI era.
Same Language Dub: The US version of the series is renarrated by American actors outside of Ringo Starr and Mark Moraghan's tenures (and even they had to rerecord many of their lines for the US version to use American terms and slang). The CGI series plays with this, while they have a voice cast for both regions, some characters use the same actors and so share recordings between both dubs.
Star-Derailing Role: Thomas and the Magic Railroad and A Simple Wish were arguably this for poor Mara Wilson. Magic Railroad ended up being her final movie before she decided to drop her film career in favor of her education.
Throw It In: Much of Neil Crone and Kevin Frank's voiceover work in Magic Railroad - particularly the banter between Splatter and Dodge. They were originally hired to improvise material for the trucks.
A few discarded stories from The Railway Series were planned for adaptation early on, but were canned due to time and budget constraints, or being considered too complex for children's stories (eg. "The Missing Coach" and "Gordon Goes Foreign"). A few other stories that were more loosely adapted are also the result of this (e.g. "Tender Engines", which was originally planned to be closer to the novel's "Tenders For Henry" until budget prevented them from making a full model for the Flying Scotsman).
A feature length special was planned that would have involved The Fat Controller loaning the railway to the king of Sodor. For whatever reason, this special was scrapped and Blue Mountain Mystery took its place. It's unknown if any elements of the scrapped special ended up being reused in any of the later specials.
Derek was meant to appear in the sixteenth season with a non-speaking role, but this never materialised. It is believed that his absence may not have been deliberate- Derek's alternative name is Paxman, and so some theorise that he was confused with Paxton when the notes were passed to Nitrogen Studios.
The show was originally planned as a BBC kids show, but after only one episode (Thomas and Gordon, obviously), the show moved to ITV instead.
Several classic characters were originally planned to be part of the core Steam Team for the show's 2004 revamp, but a need for a prominent female character, coupled with problems on some of the models, resulted in the current cast of main characters.
Duck was one of the biggest considerations, and was actually going to be part of the Steam Team, but the need for a female character overruled this.
There were considerations to keep the show in it's "transitional" form from Season 12 (ie. still using the models, but using super imposed CGI animation for expressions and human characters), however since the models became an increasing issue to maintenance budget wise, the show ultimately transitioned to full CGI.