Well, the right curves will create more drag, but I wouldn't think THAT much more for something like the BEEP. I don't have anything smaller than O72, but I can't believe that's the only factor here.
It seems like something is amiss with the BEEP.
You're saying it spins it's wheels, so you have a traction issue, not a drive power issue, right? If it's sitting on level track, does it rock? I'm wondering if all the wheels aren't in contact with the rails.