As large a radius as possible! The helix radius will determine the grade, if the helix is circular. I run a double-track helix in 5 turns on my N-scale layout. The separation between the levels and adequate clearance results in 1.67% grade in a 20-21,5" radius helix. 30 car freight trains will need at least 2-3 diesel units or a large articulated uphill. The track alignment is crucial to smooth operation, with absolutely no angulated joints.
I cut the plywood with a reciprocating saw (jigsaw). The smaller radius is 11 inch inside radius and 15 inch out side radius while the larger is 14 inch inside radius and 18 inch outside radius. I used a wooden yardstick with holes drilled at the appropriate distances to draw the arcs with pencil in the holes. It was quite simple for some with no woodworking or construction skills. Jack
Here's the key you are looking for. For each 1/4 inch rise or climb, per 1 linear foot, equates with an approximate 2% grade. My helix has a 24" radius curve on the outside and I figure the grade is right at 1.8%. Another way to say this, 2 scale feet of climb, per 100 scale feet, equals a 2% grade. You will be able to pull ten cars up the grade per one locomotive. Add a locomotive and you add ten cars. I did pull a 40 car train up my 2% grades utilizing four, four axle GP type locomotives. Pulled as sweet as ever you'd want.
I did the same thing as Hemi (a spiral helix) except my lower track was on the outside and my top track exited the inside. This allowed me to take some "030 stryrene and staple it to the side of the plywood to prevent the train from string lining to the floor in the event "something bad" happended. Mine was outside the train room. Cleaning the track was easy. If it built in the layout (under a mountain) then I would not recommend building it the way I did. The track would not be easily to get to.
