Elevation Formula

66' 8" or basically 800 linear inches.

16 inches rise, divided by 0.02 (percent grade) =800 inches in run.

Chris, no trig needed. A 2% grade will be 2" rise in a 100" run. So a 16" rise requires an 800" run, plus an allowance for vertical easements at both ends of the grade. My S scale vertical easements are 16" top and bottom for a total of 832" for a 16" rise.

Anyrail does the grades just fine, but I couldn't figure out if it would do the easement at the bottom and top, so I "eyeballed" it.  I allowed a couple of feet on either end to gradually terminate the 2.5% grade.  It obviously worked out as all my locomotives from small to huge have no problem transitioning into and off of the grade.

Ouch,800 is a bit long, even in a helix

there is a way to shorten the run.

use landings.  A landing is a straight flat section between rises like a stair landing.

the flat section lets the engine get greater traction in a short flat section so that the rises can be steeper or greater than 2% on either side.

I have used 3 -1/2  to 4- 1/2 % maybe a little steeper!

And you do need to gradually get onto the landing and out of the landing. 

16 inches is quite a bit of height to gain.    800 inches is about 67 feet.    You would need 5.31 turns around ahelix if the track was 048.     if  you go to 063 you would 4.04 loops around your helix.

If you want to know how many loops a given circle diameter will take:

Multiply the diamter (the distance across the circle 063=63 inches) by 3.14

Then divide 800 inches by the answer.   it will give how many circuits you have to make to run 800 inches and that is what  you need to get the 16 inch height.

In O gauge, a helix with a 2% grade will be a room filling structure. Using O-48 track the circumference is 150". With a 2% grade the track only climbs 3", that sure will not work. If the helix uses 1/2" plywood, no roadbed, and the track is 1/4" from bottom of ties to top of rail, that consumes 3/4" of the rise. Using O-96 track the circumference is 300", 2% grade is 6" of climb, less the 3/4" is a 5 1/4" clearance. That should be enough for most O gauge cars.

I recommend you consider a 2.5% grade and not use a helix. My layout has track from -6" to +11", a 17" climb. I wanted all 2% grades but to make it fit in the room we had to go to 2.5%. The layout is built around the walls, the six 6"climb connectors, with one exception, are hidden behind scenery adjacent to the room walls. There are six because there is an up and a down for continuous running.

So I basically have what AlanRail suggested, with the "helix" spread out to the room walls. There is a mainline at the 0, +6 and +12 elevations, the mainlines provide the flat part of the climb. It is a big climb but worth it. It is interesting to run a train that comes out of a tunnel down at river level, then with three laps around the layout is up on the high bridge.