Question on transitions to steep grades

I am building my railroad using the Richard Smith system of elevated 2x4 platforms. The area in my back yard that is the sight of my railroad has a very steep drop in elevation. From the high end to the low end 50 feet away there is a 6 foot drop. I plan to make a large portion of the railroad level and my platforms will be elevated from a low of 6 inches off the ground to a high of 3 feet off the ground. I plan to have a logging operation in the lowest area with a Bachmann Shay and Heisler pulling log loads up to the highest portion where the saw mill will be located. What is the steepest grade I can use with these engines that can pull at least three log cars and a caboose up from the tree cutting area to the saw mill? Also, how do I make the transition into the steep grade from the low level platform and out of the top of the grade to the high level platform? I don't want the engine to look awkard entering and leaving the grade section. I don't want to hit the cow catcher going up the grade or push the nose in the air comming down off the grade. Any recommendations would be appreciated. I have pictures of my area but I haven't been able to post them yet. Still trying.

John