Well, I modified my wheel sets and was able to pull one more car with the Bachmann HHP8 engine. I have 7 Amfleet cars and could only pull two with the HHP8, but after modifying them, I can pull three. I had really hoped to be able to pull more - for a more prototypical length - but I just don't see how that's possible. My layout is a flat dog bone with 11 inch curves at each end, but there isn't enough traction (even on clean rails) to pull more than three coaches. I'm seriously thinking about taking all the electric pickups out of the trucks, just to see if that solves the problem. That would be too bad since Bachmann did such a great job with the coach lighting on these cars. I guess if I want to pull a seven cars, I'll have to invest in another engine to gain tractive effort. If I put four cars on the HHP8, there's too much wheel slippage, the engine just can't overcome all the friction. Just to see how much power it would take...before I modified the Amfleet's trucks and pickup tabs, I modified a ConCor Budd Amtrak car with a rapido coupler at one end to couple with the Bmann Amfleet cars. It took two Kato P42's to pull the seven Bmann Amfleet coaches (plus the one Concor baggage car). One Kato just couldn't pull the unmodified Amfleet cars without slipping. I would guess that since I modified all the Bmann Amfleets that I could pull all seven plus the hybred coupler baggage car with only one Kato P42. The friction has been reduced, but not enough for the little HHP8 to pull more than three coaches.
As a side note...I widened the area the wheel axle has inside the wheel truck assembly. I very carefully used a file to open up the inside diameter where the wheel is held by the truck, being careful to not increase the opening that prevents the axle from falling out. That seemed to help decrease the friction on the axles. I also added a small amount of graphite where the axle touches the metal pickup and truck. This helped reduce the overall friction on the axles as well.
As a side note...I widened the area the wheel axle has inside the wheel truck assembly. I very carefully used a file to open up the inside diameter where the wheel is held by the truck, being careful to not increase the opening that prevents the axle from falling out. That seemed to help decrease the friction on the axles. I also added a small amount of graphite where the axle touches the metal pickup and truck. This helped reduce the overall friction on the axles as well.