One other consideration with a grade is whether it is a straight line or a grade built on a radius or curve. Pulling up a 3% curved grade puts a bit more load on the engine than if it were pulling the same train up a straight line 3% grade. I have two 3% grades on 24 inch radius curves. All my 6 axle diesels pull up no problem. Several of my 4 axle units need to be double headed to pull the same length train up those grades. On a previous layout I had a 3% straight line grade and could pull with only one 4 axle unit.