Three things:
1. You need a yard
2. 18" radius will always lead to frustration and disappointment. Every time.

3. Disregarding comments one and two will lead to frustration.
-I had 18" radius, and EVERY time I'd pick something up at the hobby shop, I'd think... will THIS make my radius, AND look nice doing so? Most times it was a "no", and I model the 1950's.
I have slighly smaller space, and what I managed to do with a 9X12 layout:
-24" radius MINIMUM. (4 foot diameter)

-A yard using ONE foot in width (about 5 tracks, and a caboose track) :thumb:
-lots of switching!
-A LONG mainline run! :thumb:
-gentle grades, and beautiful scenery!

-and best of all, NO ROLLING STOCK RESTRICTIONS! (from bobbers to modern autoracks, if I want!

)
I seriously think you should consider these benefits, and make a layout that would reflect these practices, perhaps a square "0" for a plan?
