Once upon a time, on the Island of Sodor, the Northwestern Railway was home to engines who spent their days being "Really Useful" by working hard pulling passenger and goods trains. But these were no ordinary engines. They were alive, and they each had names. The most famous among them was a little blue tank engine named Thomas. One day, however, the existence of the railway and the engines themselves when a diesel named Diesel 10 came in with a plan to destroy one special engine and a magic railroad that kept the Thomas and the Steam Team running.
With the help of six girls who were brought together by the magic that is carried throughout the railroad, Thomas must save his friends from destruction at the claw of Diesel 10, his driver Discord, three girls named Sunset Shimmer, Lightning Dust, and Trixie Lulamoon, and a biker named Chrysalis.
Gold Dust And Harmony is a crossover that combines elements of The Railway Series, the original, uncut version of Thomas and the Magic Railroad and My Little Pony: Equestria Girls to create a story that feels familiar and yet provides a brand new spin on the adventure, holding true to the morals that lie at the core of both movies: That magic is real and that The Power of Friendship overcomes all.
A reading of the story starts here: 
A prequel called Accidents and Promises was also made, providing some background for this fic.
This fic contains examples of these tropes:
- Adaptation Distillation: While the main story itself is based on the 1999 cut of Thomas & The Magic Railroad, it takes a few cues from the final version, with Diesel 10 having his much less frightening personality from the final cut, but with Pinchy being concealed underneath his roof until he needs it.
- Adaptation Expansion: Of Thomas and the Magic Railroad, taking the original uncut version of the movie and adding even more depth to the story. For example:
- Chrysalis, who takes the role of P.T. Boomer, the villain who was cut from the original movie because test audiences found his character to be scary for kids, is given a Freudian Excuse, which is that she lost her boyfriend Sombra in the crash that almost destroyed Lady, and she blames Shining Armor for it, since he let Sombra and some other bullies drive Lady. In turn, she wants to make Shining Armor feel the same pain she feels by destroying Lady.
- The Narrow Gauge engines, who were not seen in the original movie, help Miss Celestia as she goes to find the clue for making more of her gold dust when her supply runs out.
- Other antagonistic characters in The Railway Series, such as Devious Diesel and George the Steamroller also appear here. The latter gets involved in Diesel 10's plan, but later backs out of it because he doesn't want to hurt anyone.
- Each of the Mane Six other than Twilight is a friend of a particular member of the Steam Team for a trait of their personality that they have in common.
- In the original film, all the engines' drivers and firemen were not present and the vehicles seemed to be acting completely on their own. In the fic, the crew members are present. Notably, Rainbow Dash takes over as Gordon's fireman when his normal one gets sick and the relief man is unable to make it. Also, Discord is revealed to be the driver of Diesel 10. And during the climactic chase scene, Spike helps save Thomas from Diesel 10 by opening his regulator to maximum.
- Along with several other standard gauge engines, Edward the Blue Engine also appears. In the fic, it's revealed that he knew about Lady and where she was hidden, as he had helped Shining Armor hide her after she had crashed. Later, after stopping Diesel 10's crew and getting himself wrecked, Lady returns the favor by using her magic to fix him.
- Discord is a Gadgeteer Genius who created the hydraulic claw that Diesel 10 calls Pinchy.
- Adapted Out: While gold dust, Lady, and Muffle Mountain are all still integral to the fic's plot, Shining Time Station is not mentioned at all. Furthermore, any scenes that take place around Shining Time instead take place on Sodor, with most of the major human characters' roles being assumed by the Equestria Girls characters.
- Ascended Extra: Sir Topham Hatt was The Ghost in the movie, only being mentioned a few times and as a garbled voice over the phone. Here, however, he breaks up the argument between James and Diesel 10 at the sheds and telling the latter to behave when he's awak. And later, when he returns, he ensures that Diesel 10, Sunset, Trixie, Lighting, and the other antagonistic characters are punished accordingly for causing trouble on his Railway.
- The Cameo: Despite appearing multiple times, many of the engines in this fic function as this, since only a handful of them actually have significance. These include Boco, Diesel, Trevor and others.
- Character Narrator: Twilight, who assumes the role of Lily in the story, serves as its narrator. At the end, it's revealed that she's been narrating the story not only to the audience, but also to her children, Dusk Charger and Velvet Glow.
- Earn Your Happy Ending: After having faced a number of hardships on their journey together, all the main protagonists come away much happier and better off than before. For instance:
- Twilight and the other five girls discover the importance of friendship. Twilight also starts dating Flash and later has two children with him.
- Celestia and Luna get their gold dust back after having deciphered the clue to its source.
- Shining Armor and Cadence, who spent most of the story fighting to protect Lady from Chrysalis and Diesel 10, are not only able to share a ride with Lady on the magic railroad, but also move to Sodor so they can stay close to the railway with Lady. Furthermore, the former eventually lands a job with Sir Topham Hatt and the Northwestern Railway while the latter learns to use gold dust like her aunts.
- Lady, after having spent decades being hidden away in Shining Armor's workshop, is finally restored to life and is able to run her railroad once again.
- Easily Forgiven: In the end, George, Splatter, and Dodge get off unpunished for betraying Diesel 10, with the former confessing his involvement and the latter two heading to work at the Scrapyards, replacing Arry and Bert as they were meant to do when Sir Topham Hatt brought them to Sodor.
- Even Evil Has Standards: Towards the end, many of Diesel 10's allies eventually abandon him because of his extremism towards steam engines among other reasons. For example:
- George the Steamroller leaves Diesel 10's fold by refusing to destroy the magic buffers, as while he hates railways and wishes they'd be ripped up and turned into roads, he actually doesn't want to actually hurt the engines.
- Devious Diesel also abandons Diesel 10 because he doesn't want to inflict harm upon the Steam Team either.
- Everyone Calls Him "Barkeep": Most of the Equestria Girls characters' names are explained to be nicknames that everyone prefers to address them by in place of their real names. And, of course, the engines call Sir Topham Hatt the Fat Controller.
- Rainbow Dash: Rayne Dash
- Applejack: Jacqueline Apple-Smith
- Fluttershy: Florence Shelley
- Rarity: Aurora Belle
- Pinkie Pie: Pinkamena Diane Pie
- Twilight Sparkle: Twyla Sparkle
- Chrysalis: Quinn Chryssa
- Shining Armor: Simon Armstrong Sparkle
- Cadence: Mi Amore Cadenza Sparkle
- Eviler Than Thou: Chrysalis is this to both Diesel 10 and Discord. Discord even lampshades this at the story's end, where he calls out Chrysalis for being insane when she attacked him.
- Freudian Excuse: Chrysalis' rivalry with Shining Armor is born from her hatred for him after his engine, Lady, suffered an accident that killed her boyfriend Sombra, and she wants to destroy Lady to make him suffer for indirectly causing the death of her boyfriend.
- Gadgeteer Genius: Discord, who modified Diesel 10 with his claw "Pinchy".Tpin
- Official Couple: Twilight and Flash. They even have two children together in the epilogue.
- Shout-Out: Chrysalis quotes "Back to the Future" after she, Diesel 10, and Discord fall from the crumbling viaduct and into a passing barge full of trash.
- Spared by the Adaptation: In Magic Railroad, Burnett's childhood friend Tasha had long since passed when he finally fixed Lady. In this story, Cadence, who takes the role of Tasha, is quite alive and well, meaning that Shining Armor is able to keep his promise to take her on a ride with Lady.
- Laser-Guided Karma: Many of the villains get their comeuppance. For example:
- Chrysalis and Discord get arrested and sent to jail, with the former getting a more lengthy sentence.
- Diesel 10 is sent away by Sir Topham Hatt.
- Sunset, Lightning, and Trixie are sentenced to repair all the buffers they smashed when looking for the ones that served as the portal to Lady's railway. It's also punishment for stealing George, which the steamroller's driver was less than pleased with.
- Arry, Bert, Diesel, and Bowler are all sent away for making an unauthorized return to Sodor.
- Mythology Gag: In the epilogue, Thomas has a flat running plate like the one his The Railway Series counterpart got after an accident.
- When Sir Topham Hatt returns, he reprimands Sunset and her fellow delinquents for causing "confusion and delay", which was a common thing for him to say a lot whenever the engines do something wrong.
- Also, when Discord is introduced as the driver of Diesel 10, he asks, "Who were you expecting, George Carlin?" On Shining Time Station, Carlin was the second actor, after Ringo Starr, who portrayed Mr. Conductor, whose role in the fic is taken by Celestia. Carlin was also the narrator for the American dubs of the first four seasons ofThomas the Tank Engine before he was replaced by Alec Baldwin in the fifth.
- Twilight's being the narrator is a nod to the original 1999 script, where an adult Lily and Patch narrate the story to the audience.
- Secret Keeper: Edward, who helped hide Lady after her first crash, kept her location a secret from all the other engines.
- Villainous Breakdown: Diesel 10 gets one at the climax when all his allies abandon him because Everyone Has Standards.
- Chrysalis has a similar moment when she falls into the barge full of sludge, beating up Discord out of pure rage until the two are broken up by the police.
- "Where Are They Now?" Epilogue: The final chapter features one with Twilight narrating the fates of all the main characters, including the engines and the girls, as well as a scene showing Twilight telling her and Flash's children the story, and showing them a scrapbook of pictures from their earlier years.