The usual fix is to place washers on either end of the worm gear in the trucks so as to reduce to a minimum the forward and backward motion of the worm gear. Not so much as to introduce any bind in the worm gear's rotation, but so that it will only slide back and forth a few thousandths. The surging comes about from the wheels trying to push the gears faster than the motor is running them, so they push the worm gear to one extreme end, and they momentarily bind up, the locomotive bucks, this reduces the binding, and the cycle repeats.
I agree but a more basic question is, are these Kato Stewarts or the more recent home-made drives? I have a lot of Stewart F units, and I've never seen them bind up on a downgrade. But maybe my freight cars are not as free rolling as Earl's must be...
