Yeah, it's interesting that way... no matter where the line is drawn, there is always an example that blurs that line again.
Maunch Chunk Railway (may it RIP) fit basically all of the requirements to be considered a roller coaster. Not built to serve strictly as an amusement ride, but served as one from its very first run. Does that make it less than a roller coaster, or more than one? I'd say it was at least a roller coaster, which should qualify it for designations like hyper-, giga-, tera-. Its top speed, depending on which source one believes, was as high as 65 mph, though some claims suggested higher. We probably will never know what its fastest run truly was, as such stats were played fast and loose. Arguably it was braked below its achievable top speed on every single run, further complicating things. But with the more amazing claims tossed, it still approached, and perhaps achieved, hyper- territory for speed.
So I'm not sure about the speed aspect as a qualifier, since I've never seen that in the definitions -- save for the spirit of the thing, which seems impossible to incorporate into a tight definition anyway. I'd classify Maunch Chunk as whatever stupid nickname we will someday give to roller coasters with elevation changes of more than 600 feet -- exacoaster, presumably -- even though its top speed probably didn't quite reach that of a hypercoaster. That's just my opinion, but I really like the idea that it was the world's first and only exacoaster.
If it did top 70mph, though, even once, how would a speed-tied definition classify it? Exa-, but demoted to hyper- due to speed? Or exa- because the strict definition doesn't really consider speed? Or neither because the speed disqualifies it completely? Gaaaaah.
If someone could please force the issue by building a roller coaster that is basically a gently undulating full-circuit ramp 200+ feet tall and over a mile long, it would be greatly appreciated. I understand they tried this in the 1990s at Blackpool Pleasure Beach, but messed up when they made the very first undulation a bit too steep, so it didn't count.