My thoughts, which if you add a buck will get you a coffee at Mickey D's, LOL.
Radius: Make it as LARGE as possible.
An around the wall LOOP with a lift out or tilt up for entry into your room. Switching will be far more interesting than running around in circles, but I am guessing there will be times you might just like to sit and watch stuff roll, and with 20 foot sides you won't likely be chasing your tail.
Keep stuff within your reach.
A turntable one end and wye the other.
2 or 3 track main and at least one yard..
Good Luck!
Simon