Pals and Friends
This engine is unique for being the only character to ever be the narrator of an episode of the show.
Thomas
Engines gesturing by will or "Bouncing"
While many engines don't believe in the supernatural when it comes to things like gremlins, monsters and so on, they do make one exception. Many engines seem to unanimously agree on the existence of this supernatural figure, and we've seen this figure before too! Once in the classic series, once in the CGI series, and once a year, every year, in our hearts.
Santa Claus!
This Thomas special is quite the strange one, as it's the only one to have (multiple) educational learning segments that interrupt the main plot of the movie.
Calling All Engines!
While this company is best known among Thomas fans for making a great variety of high quality Thomas items, others know this company best for releasing a variety of high quality made model tractors by companies such as John Deere.
ERTL
While Thomas is number one, his basis has never had an engine that carries that number. However, there are other characters who have basises that carry said number, like this one (only one angine/answer needed for points).
Emily and Skarloey
The CGI era seems to have a pattern of disregarding railway realism. This change in railway realism was huge, as it made engines look ridiculously unrealistic as they climbed up big slopes and gradients.
The steepness of Gordon's Hill
Aside from Thomas, there have been several confirmed winners of the Great Railway Show in some competition, whether it be in The Great Race or not. These are just a few examples (must name 3 for points).
Carlos, Asima, Ettiene, Rajiv
This Thomas special is unique for having its own special-exclusive narrator that has not narrated any other piece of official Thomas media. This excludes unreleased footage of seasons released around the same time as this special.
The Great Discovery
Early Engineers, Capsule Plarail, My First Thomas/Golden Bear,
Aside from Donald and Douglas, these 3 engines are the only 3 members of the steam team to have the exact same wheel configuration.
Thomas, Duck, and Toby
There are many engines who, while they have traveled on British standard gauge track in Thomas media, do not have basises that run on the aforementioned gauge. These are just a couple examples (must name 2 for points).
Hiro, Nia, Sam, etc
The CGI episode No Steam Without Coal has taught us that there can't be any steam without coal. Except for these four steam engines, who use oil as part of their way of creating steam (must name all 4 for points).
Timothy, Bash, Dash, and Ferdinand
This Thomas special was the first ever to be shown in theaters non-consecutively, with the second one being a one day exclusive release specifically in the United States.
Thomas and the Magic Railroad
Many popular Thomas ranges have a habit of making metallic, golden, and even silver versions of Thomas characters. These two are just some examples of the many that don't have any of those 3 types of characters (must name 2+ for points)
Bachmann, De Agostini, Nakayoshi, Thomas WOOD, Thomas Wooden Railway (Revamp), Early Engineers, Interactive Learning Railway, etc
These 3 engines part of the original 11 have carried the numbers of existing members of their class in The Railway Series (must name all 3 for points).
Toby, Duck, and Oliver
While Spencer is best known for his streamlining, many don't see the big problem with him: his whistle. His whistle is not only not an A4 whistle, but isn't even from the LNER. Instead, it is a stock sound effect of a steam whistle of an engine belonging to this railway.
There are many gauges on Sodor, such as Snowdin narrow Gauge, Talyllyn narrow gauge, British standard gauge, and miniature gauge. However, there is one "large" pictured outlier that does not fit into any of these categories of gauges, and they even have a face!
Iron Duke (RWS)
These two Thomas specials are the only two to have exclamation marks in the specials' titles.
Calling All Engines! and Big World! Big Adventures
Golden Bear/My First Thomas
While Henry has gained a dome and Gordon has lost one, this character is the only one to have his dome's location changed (when comparing Thomas universes).
James
A british railway rule where a train driver must have his fireman, guard, or etc alert the nearest signalman of the engine's presence after stopping the train at a signal.
There are many twins on Sodor, like Donald and Douglas, Bill and Ben, Bash and Dash, and even Skarloey and Talyllyn. But there are two male twin characters who, while they have both appeared on screen before, they've never been seen together. And while only one member of their class exists in real life, two of them have been on/are in Sodor.
Dennis and Norman
This Thomas special holds the record for longest Thomas special released.
Thomas and the Magic Railroad (Rerelease)
Interestingly enough, the people behind Thomas promoted their own merchandise in official Thomas episodes. In one of the TVS episodes (which must be named), this specific ERTL item is seen under the bed of a sleeping boy. (name both the ERTL item and episode title)
ERTL Percy in Flour Power