I've heard them just call out to the "regular" line to find single riders (who then got to get out of line and into a ride vehicle) at Space Mountain and at Splash Mountain, and also at Tower of Terror. I know this because dd won't go on any of those rides so dh and I have to take turns riding them unless we have friends/family with us.
When we were at Test Track in October, it seemed like the single rider line was incredibly long, and really didn't appear to be going much faster than the regular line. (which, thankfully, dd was willing to ride this time so the whole family was in line together!) This was at a time when the standby line had a very long wait. (but dd was determined! LOL)