Kraken didn't have a single-rider line when I was there, but they would call out for single riders to come up and fill in spots.
Funny story: I rode by myself as my girlfriend is prone to motion sickness. The very first time I rode it I was in line for a while before I realized I was in the front-row queue. I decided to stay there even though it moved much slower and it was awesome as a first time experience!
Anyway, the girlfriend meanwhile is waiting out front watching people get in line and then exit with no sign from me...she thought I must have fallen off!