I have added thin strips of lead to the underside of my flat and open wagons.? Fixed to the floor wither side of the central chassis
member but thin enough to clear the back of the wheels.? I have added very thin lead sheet to the inside of skip bodies but I don’t tip mine.? You might be able to add some liquid lead to the underside of the frames but it would be a fiddle.
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A rake of three tub wagons weighs 30g so about 10g per wagon.