There were two versions. The Series I has the small flat sided motor, the Series II has the larger Mabuchi round motor. The small flat sided motor is pretty weak, and any significant running with load will kill it. I've heard conflicting reports on the possibility of putting the Series II motor into a Series I chassis, but I believe it's not easily done as the mounting is different.
I'd consider looking for a Series II locomotive and just swapping out the one that keeps eating motors.