Yeah I wonder if the casting of the boiler or frame isn’t done properly if maybe the rear is heavier than the front?
I wonder if it’s an occasional problem that only a few have, or if all of the units have a poor CoG and only a few folks pull heavy enough consists to notice.
As far as solving it without removing the smoke unit, I’ve always been a fan of lead sheeting. You can tuck that stuff n a lot of places to add weight. It’s good you got the amount of weight you did in there, but I wonder if all 10 ounces was needed.
I’ve had similar problems with HO locomotives but never O-scale ones. Something I will keep in mind.