Bachmann On30 4-6-0 & Grades
I tested the Bachmann 4-6-0 with six driven wheels on a very slight grade pulling three cars. The grade was less than 2% with shimming my platform slightly and it slips to about stopping at slower (not crawling) speeds. The motor has plenty of torque spinning the wheels. It is not an issue for me as my track is flat.
Then, I used a Walthers Mainline HO engine with 12 wheel drive pulling the same cars and it did not slip at all at slow speeds on slightly less than 2% grade. The Walthers engine is a little heavier vs the Bachmann.
If you want On30 engine used on a grade, you will have to seek another manufacture besides Bachmann. Perhaps it is my NOS brass track and newer nickel/silver track provides more traction.