Hi Bob,
Usually when work with such tight radii, reality has to take a back seat to functionality. Atlas went with 9 inch diameter increments for their entire track system, which translates into 4.5 inch spacing (center rail to center rail) on concentric curves. Back in the mid 90's when Atlas introduced the track there were still relatively few articulated steamers on the market and even fewer scale passenger cars and modern freight cars. That meant most people could get away with 4.5 inch spacing without clearance issues. Unfortunately, unless you avoid running anything bigger than an F-unit for power and 40ft. box cars, the O-54/O-45 spacing is just too tight. That's par of why MTH stuck with the traditional Lionel tubular sizes (31, 42, 54, 72) for RealTrax and ScaleTrax. When Lionel designed FasTrack they used concentric curves like Atlas, but went for 12 inch diameter increments (36, 48, 60, 72). That works out much better when a Railking or Lionmaster Big Boy passes an 18 inch passenger car.
If you goal is to improve the look of the track spacing I would suggest adding to your ground cover or other scenery details to male the two tracks appear as separate mainlines. A little ground foam and lichen can go a long way.
