RE: Thirteen - that currently needs 18(?) people per train or zero. I once witnessed them spend several minutes reshuffling people when they ended with 17 on a train after someone had to do the walk of shame (they already had 1 group of 3 from main queue and a group of 3 from the exit who hadn't been paired up). They had to open all the restraints, get people to move seats and then get 2 more people on from the loading bays.
If Thirteen is to open they're probably going to have to ban single riders from it entirely, odd numbered groups could also be a problem as you'd be limited to 2 of them per train unless they're willing to abandon their loading policies.