Rail Truck woes

[Flare]

So yesterday I bought a factory sealed rail truck, and of course the first thing it did on the programming track was split a gear.

The trouble is I have two others and they all seem to have the same problem:  Resistance in the drive line.

The motors spin freely when I disconnect the drive shaft or run them without the differential cover installed.  But when fully assembled they either make a grinding noise or don't run at all.

Before I disassemble this new truck and hope the white plastic or metal gears I've tried in the other two work better, does anybody have advice for resolving the problem?