He came to Sodor immediately after recieving the news that Gordon got very ill. This was before any of the engines fully converted into beasts. After some insistence from Gordon that he was okay, he headed back. But once he reached London, he began acting weird. His crew, heavily suspecting that he contracted the infection, immediately placed him into a shed to isolate him from any other engines. Their speculations were soon proven correct after he entered stage 3.
Miraculously, FS was a bit harmless for the most part after he transformed, but he was very afraid and confused at what had happened to his body. He instantly bolted out of the shed the moment he saw those doors open, accidentally trampling on a few workers and throwing off anyone that tried to get in his cab like a frightened animal.
It took around five weeks to find him, and professional catch-and-release hunters were hired to hunt, tranquilize and bring him back to his shed. Over those weeks, he had been eating wood and small critters to satisfy his new stomach. Luckily, he wasn't spotted in many public spaces, but BBC news had TONS to say about it. FS eventually calmed down and is doing much better. He's still a nuisance to Gordon, even if he's a beast now. It was also discovered he was trying to get to Sodor. FS still won't say why he was trying to get there.
My au was originally gonna be a Sodor Fallout au (thank god I didn't go down THAT direction), with all the engines being Choo Choo Charles-esque spider trains. I later started making them more fluffy and cuddly looking and just did my own thing at that point.
Here are early concepts of Thomas and Percy's faces, who got goofy nicknames
Tommy Two-face randomly goes back and forth between friendly and hostile. He’s unpredictable.
Mandible Percival is partially aware of his surroundings and the people he attacks, but not all the time. His entire lower jaw rotted and fell off.
In a general sense, who was infected via bite/scratch ratter than getting the suspicious coal?
Salty - got attacked by a feral Porter. He actually doesn't mind being a reptile-like beast. It means he can go visit Rusty and crawl up Cranky's support beams to talk to him better, plus, he can swim!
Diesel - got scratched up by a feral Duck. Duck suddenly remembered the beef he had with him and just went up and scratched him, then left. He refused to elaborate.
Mavis and Daisy - got cornered by Thomas, Percy and Toby while trying to escape Ffarqhuarr.
Bertie - Got bitten while trying to keep his passengers safe.
Annie and Clarabelle - couldn't go anywhere and got a bit scratched up.
Henrietta - was attacked by a feral Toby. Tried reasoning with him, but it was too late.
Carolyn - was chased around a bit, but escaped. If she could, she would spray water at them. She now treats them like big kittens whenever they act all adorable, and treats Diesel 10 like a dinosaur if she ever sees him.
Cleo - managed to hide from the infected. She's still secretly afraid of them, and has to have Ruth reassure her that they're still the same engines she got to know before.
Ace - escaped, he still treats the engines and infected road vehicles as his friends, but he also treats them like Australian wildlife, and sometimes just pisses one of them off to see what happens. He tried that with Edward to see if he could get on the old guy's nerves. Spoiler; it worked after the 1000th time.
Paxton - got jumped by the NG engines, all at once. He's pretty unnerved now when he goes to the BM quarry and literally squeaks when an engine just suddenly pops out of one of the tunnels without warning. He also struggles to understand Rusty. That little diesel just sometimes spawns everywhere Paxton looks. Rusty's an definite enigma...