Some rolling stock, particularly older freight cars, rock back and forth as if off the track or running on lopsided wheels. If so, the trucks may not be properly bolstered.
Here is a simple idea to solve the problem. Mount one truck on the car with its mounting screw only. But build up the other end of the car up with bolsters that almost, but not quite, touch the truck. The bolsters are just styrene strips built into a triangular tube.
This method 3-point truck mounting method should eliminate the wobble.