I think the Coke train may have been the most colorful marketing type FANTAsy train, lol. That kind of. stuff goes back to the tin Baby Ruth's and Kusan beer car line for collecting at least, which looks great on display surrounded by family steins. And that's where they shine; on a shelf. Especially the more celebratory marketing things like the State, Presidential, and holiday cars too. Ie those sets are cool to look at because of the variations, but for running I'd rather have six Heinz pickle vat cars than variations of vat cars, or six Baby Ruth vs a Hershey train ,etc.
I think that just about anyone into trains, even scale folks, could be lured into at least one fantasy train if the right theme was offered. The right team, band, softdrink, hard drink, candy, tractor, automobile, or support ribbon has landed on more than a couple scale layouts lol, or at least the shelves That's why they are made, we are into marketing. If we weren't, you wouldn't care what road name was being made on that new release.
The crazy stuff is just plain fun. Running them is play, which I'm proud I can still do at my age. Sure it's on the fringe of not being any kind of train at all, but even that's subject to personal perspective
I also think the fantasy train is an industry must, needed for kids. Without the more whimsically fun cars like the chicken sweep, gunfighter, rocket launcher, Mercury set etc, that I had when young, who knows, I might only have a slot car layout today. It's likely Thomas and Chuggington that will bring the majority of next generations into the hobby. Not today, but when they are our age, that scale loco is going to catch their eye a little easier having run Thomas to death as a youth. So don't wish them out of existence thinking your selection of scale will grow that way; I fear the worst, should it become true.
I was about to list my favorites, but it would take way too long. I think if I could only have one engine, one car, and a caboose and it had to be fantasy, it would be the fire locomotive, ladder car, and red Cross work caboose. My "wish" would be an engine suited to go with the old Bugs Bunny / Marvin the Martian cars. I built one till then