There are a number of CAD packages to help with layout design, my favorite is AnyRail, which has a free demo available. However, I've found that designing a layout from scratch is easier using pencil and paper, and then using the CAD program to nail things down. It's still easier to change ideas and concepts on paper, and then the CAD can verify curve radii, check clearances, and even generate a bill of
materials.
-Mark