Ever decreasing concentric circles

I know with Lionel FasTrack (36-48-60-72) that 12" as in your method, the O36 and O72 switches when mated turnout to turnout keep things spaced properly.  This is one less oval than you had, but keeps you in one track manufacturer and spaced properly.

Actually, that's 4.5" between track center lines. O-72 refers to the diameter as opposed to the radius. You won't be able to match that spacing with any curve-to-curve arrangement of turnouts. If you used Atlas #5 turnouts on the straight sections, you could pull it off but you'd need a minimum of 60" of straight space to pull it off properly -- 40" for the turnouts plus an additional 20" of straight track to prevent taking a "reverse curve" at mainline speed. Also note that any large equipment would have overhand/underhand on curves and could hit another train on the adjacent track, so short equipment is the order of the day here.

FasTrack spaces 6" on center, but making up something like this would be tricky because the turnouts are all curve-replacements, have inconsistent straight leg lengths and aren't available in all sizes as I recall.

That's kind of how I had it figured.  In fact, as I have come up with no satisfactory solution, my next query will be going down from O72 to O54, because I can probably do that with a transition piece between the different track systems.  Only reason I want to try this is because I have a lot of O72 track and switches, but in order to do it will have to use expensive alternatives.  MTH scaletrax would be a perfect fit for the O54 section.

What kind of track do you have? Is that same track available in O-54?

O-72 to O-54 concentric loops is a bit easier as you're dealing with a 9" spacing between track centers, which you can pull off using O-72 turnouts and some cut pieces.

I have a lot of K-line super snap O72 and straights, because it was cheap and good looking.  That's why I made the post, but now am considering something else.  I have two reversing loops, but want to add another, but now may do O-63 with yard in the center.  I need more places to park or idle trains.