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Could it be The Mystery at Disney World by Carole Marsh?
http://www.amazon.com/Mystery-Walt-Disney-World-Places/dp/0635021048
No, that's not it. One other odd detail that I remember was that, in the only story that had anything to do with Space Mountain, Disney World was never said by name.
There's a Little Golden Book based off Space Mountain, but it doesn't seem like what you're looking for.
This probably isn't the one you asked about either, but: https://www.goodreads.com/book/show/20784990-space-mountain
A small anthology of sci-fi stories based loosely around - get this - Space Mountain. Well, the first story was, anyway; it involved a kid riding the coaster and getting stranded on an alien world. If I remember correctly, the rest were basic kids' sci-fi fare. Anyway, I read this book as a Disney-loving kid, and I just remembered it now, but I can't quite recall the name of it.
Edited by TheToonGeekette