if you read thru this thread, you'll note that (in my opinion) Fiesta is notorious for running only one train on busy days (to up the Fast Pass sales, I'd guess).
the good thing is that Fiesta does have some really fun flats, and for the most part, the coaster lines move fast
(tho on a busy day, I've seen waits on Pandemonium and Scooby Doo reach well over 2 hours).
Any coasters/rides with single rider lines?
Pandemonium/Tony Hawk used to, but they have redone the line since Halloween. (There was a halloween maze in the line this past year). So I'm not 100% they still have the single rider line.