This is such a specific rabbit hole to go down, and I’m all for it. With Thomas and Percy, the dynamic is usually built on their contrasting personalities – Thomas is the cheeky, adventurous, sometimes reckless one, while Percy is the dependable, earnest, and slightly anxious little engine. That clash is a playground for fanfic writers.
A huge trope I see is the 'Accidental Confession' or 'In Vino Veritas' scenario. Thomas brags about some daredevil stunt he pulled, Percy gets worried and scolds him, and it spirals into a heated argument where one of them blurts out something they didn't mean to. It's a classic because it fits their canon interactions so well; Percy's fussing is a cover for concern, and Thomas's bravado hides a need for approval.
Then there's the 'Comfort After a Crash' trope. Thomas gets damaged, maybe from a prank gone wrong or pushing himself too hard, and Percy is the one who stays with him in the shed, bringing him coal and water. It flips their usual dynamic – Percy becomes the caretaker, and Thomas has to be vulnerable. Those stories often have a quiet, tender tone that really highlights their bond. I've also seen a fair share of 'Rivals to Lovers' takes, where they're competing for the title of the Fat Controller's favorite or something, but the rivalry feels more like playful bickering than genuine hostility. It rarely gets truly angsty, which I appreciate; the fandom seems to prefer keeping their conflicts low-stakes and heartwarming.
Less common but delightful are the 'Ghost Train' or supernatural AUs. Percy sees something spooky in the foggy yards, Thomas doesn't believe him but investigates to prove him wrong, and they end up having a shared spooky adventure. It's a fun way to explore the setting beyond the sunny days of Sodor.
Honestly, the tropes aren't usually about grand drama. They're about small moments – sharing a goods van, getting caught in the rain, Percy patiently listening to Thomas's latest scheme. The appeal is in the cozy familiarity and the unspoken affection that builds through these everyday railway mishaps. It's comfort food in fic form.