I didn't notice an official single rider line when I was there, they were just asking for single riders out of the standby line. Where were you doing Single Rider?
Fastlane line.
How does that exactly work though?
In my experience, while single riders and Fast Lane holders share the same line, people with Fast Lane are given priority. I had Fast Lane and they were consistently seating me before the single riders ahead of me in line and also asking which row I wanted. It appears they just use the single riders to fill in the gaps... which is how a single rider line should work.