You can also "cheat" a little in ho'3 since narrow gauge railroads would generally run a little steeper grade than standard gauge, and most of the narrow gauge trains were short, 4-5 cars max. Most locos will handle 4-5% grades, although they may jerk a bit coming down something that steep. If you try to run a class one mainline that steep, you will find yourself having to use way too much power for the size of train your pulling.