You might consider adding some weight to the powered unit. I have done that for a few locomotives, especially diesels. It's amazing what a few ounces sometimes will do.
I added just over a pound to this HHP-8, and it almost doubled it's pulling power! Before the weight, with six or seven 18" passenger cars, it would spin in any grades or sharp curves. I'll pull a dozen now, and never missed a beat. It used to weigh around 4 pounds, the extra pound has an amazing effect!