Do you have a traction problem? How many cars are you pulling when it starts to slip? How much grade are you pulling? Are you still using the traction wheels with the rubber tires and are the tires in good shape? I have an SD 70 and have changed out the traction wheels to metal wheels. So far I have towed 10 cars with it and not had any traction problems. I am going to increase it to 20 in increments of 5 and see if I still have traction. I will probably keep increasing the consist until it starts to slip. Then I will make the decision if I should have another engine or add weight. If I start to have traction problems I will probably start with 2 lb and increase it if needed. I plan on using shotgun shot and mixing it with epoxy and putting it in the belly tank on each end so as not to interfere with the speaker. I will make a bulkhead on each end to contain the slurry until it sets up. Hopefully i won't need it as I can see a potential domino effect with the four small screws that hold the belly tank on. I may have to use larger screws and mount some metal threaded nuts on the frame so they don't pull out. I do like my SD 70 it is a very handsome engine. Mine is in the BNSF orange/green std color scheme.