Are saying you want to have 18" radius curves inside your helix? If so, you might want to rethink that. Even with smaller engines and cars, 18" is a tight radious on the level. As you start to rise, physics takes over and more flanges pressing agains the outside curved rail begins to increase drag. As drag increases, two things happen. Motive power that pulls a train at the bottom of the helix will stall before the train gets to the top of the helix. The other problem is called stringlining, where the train at the top of the second level is now exerting less force on the cars behind it than on the cars still making thier way up the helix. At some point, this differential starts to move some cars faster than others and slack action between the couplers pulls cars off the track. Coming down a helix makes the issues worse, as cars at the top want to push on cars entering the helix.
It looks like, from the size of your track plan, that you have enough run available to get to a second level without using a helix. If you do use a helix, make one with the broadest curves possible.
