Steamers have long wheelbases under the bottom dead centers of those four drivers on each side, particularly those engines designed to move loads at speed, as the Berks were. If your pilot truck is lifting out of the outside rail on curves, it can be due to insufficient weight/spring tension to keep it down, or as likely, it could be a result of the rear two driver axles lifting the frame on one side over uneven rails. If you have a superelevated outer rail, say, but somewhere most of the way around the curve that outer rail dips unbeknownst to you, the frame will rise on the one side in compliance, but the outer wheels on the truck already running over the dip may be inclined to lift and cause the derailment.
One other cause could be flashing, crud, or some other mechanical impediment between the truck and its under-frame bearing or pivot point.
This is all assuming your curves are not in the 18" range in HO. Larger drivered steamers with four axles will do quite a bit better on 22" and wider curves.
What you must do, after you remove and inspect the bearing surfaces on the truck and the frame with your forefinger, and ensure it is lubed, is to set the loco properly on the rails and run it slowly through the place where it wants to derail. Note where the truck lifts. There is a dip there that must be mitigated a bit, perhaps only by 0.5 mm. Lift it. Alternatively, note if the loco cants to the opposite side on high rails if you have any superelevation at all. You may have to reduce it.