I have recently been hearing occasional reports about split gears on both AM ans SHS products. It doesn't sound like an epidemic, but it has been happening elsewhere. Northwest Short Line has a replacement gear for the SHS 2-8-0's that someone commissioned.
The interesting thing is my Scale U25B's have a lot more "mileage" under their wheels than the HiRail units. Maybe just the luck of the draw.
Rusty