I doubt that adding a spring to these accomplishes anything. I did have some of the original GN cars as well as original Corrugated sets. IIRC, I upgraded all of the GN cars using the free upgrades from Kato. If you're going to keep these, you will probably want to fully upgrade them all with new Kato couplers, covers and springs. It takes only a couple minutes per car and brings them fully up to date.