NW,
I believe you may have it backwards. My understanding is that the most popular software will let you design your own layout, and then tell you exactly what pieces you need. I'm not sure if the software works in reverse. Others will soon advise on this.
If you already have a large set of track sections, then get down on the floor and start hooking them together to see what layouts are possible. (Pretend they are legos). It's fun. It's called "building your own layout. " :-)
Mannyrock