The others got it on how the Mine Train system works. It's a way to handle odd-numbered groups without needing the space to run a dedicated Single Rider queue.
The problem with what I experienced at RnRC was that rather than grouping a party of 3 and filling them in with a Single Rider (which is how Single Rider works at all of the attractions that have it), the grouper was then asking the size of the next party in the main queue. If it was odd-numbered, they'd have the odd person out fill in the available seat with the party before. Sometimes if the next group was a small even-numbered group she'd even stick them at the back of the train to get them out of the way, hoping that the group behind *them* were an odd-number.
It was very odd, almost like she didn't want to use the Single Rider unless absolutely necessary. In fact, I think I recall seeing people who'd gotten in the Standby queue the same time we entered the Singles queue actually *pass* us near the boarding platform and ride before we did.
-Rob