12'x20" layout questions

Modernly, it's measured to the center rail. It used to be pretty inconsistent -- sometimes measured to the outer rail; sometimes measured to the outside of the ties.

If you want to run multiple trains conventionally, you should have good transformers with a separate throttle for each mainline.

You can measure the diameter from anywhere as long as you measure to the same position on the other track.  Ex. center rail to center rail,  outside rail to outside rail,  end of tie to end of tie.

Rick

Please don't go for separate circles.  You will get bored and lose interest swiftly.  There are numerous awesome people right here on this forum that can design track plans that will greatly increase the interest and appeal of a track plan.  With rare exception, people rarely stick with separated circles.

Come on man!  Do better.  Put forth a little creative energy.  In the end, you'll be very glad that you did.

A lot depends on what you enjoy most.  If having 4-6 trains operating simultaneously and just enjoying the visual and auditory experience is what you're after, loops are fine, particularly for conventional mode.  You can have trains going in opposite directions, fewer derailments and varied speeds.   No switches needed.

If you want to use operating accessories, a yard or two with switches and bumpers is a plus.  If you want to simulate real train operations, you'll need a layout that mimics the real world, and switches are essential.  There may be a happy medium, with 3-4 interconnected loops (or maybe 2 loops interconneted) with switches and maybe a yard or two for storage, operating accessories, etc.

If you don't build it for permanence (hold off on the scenery, ballasting, etc.) you can always change the plan.  But for around the wall layouts, loops work pretty well, as you cannot get much depth unless you insert an island jutting into the room.  There's something mesmerizing about 4-5 trains running at once, but you may, as suggested, get bored with that.

sound like a plan, go for it. However lop running can get pretty boring so plan you layout as a stage for your trains and adorn it with either nice scenery or lots of accessories which will add to your pleasure. 

Also don't discount the world of today and adding either Legacy or DCS. The trains coming out today are really feature rich and add another dimension. With your plan these can be added on in the futire

Enjoy the hobby

Concur with above!  And use the widest biggest curves and turnouts as possible as space permits.