Atlas Track Question

Hello Nicholas. I’m not sure if your specific question is answered in it, but I found the following to be useful when I converted my Fastrack layout to Atlas a couple years ago. Good luck with your design. 

http://download.atlasrr.com/Tr...-132%20O%20Track.pdf

Atlas measures their track diameter between the center rails.

Jan

Here's Atlas O81 inside O90.

O-90 fit nicely within 8'. I have 3 concentric curves; O-90/O-81/O-72

Depends on how close you like to run trains from the edge of the table. From the middle of the center rail to the outer edge of the ties is about 1 1/8”.  From the middle of the center rail to the outer edge of a switch machine is about 2 1/2”.  So 96” - (O90 + 2 x 1 1/8”) = 3 1/4”.  Divide by 2 gives about 1 1/2” from the edge of the table to outer ties.  If there is a switch machine on the outer edge it is about 1/2” from the edge of the table.  You would have to decide if that is running trains too close to the edge.  You don’t have to use all the same radius curves.  For example, an outer half circle of O90, O90, O81, O81, O81, O81, O90, O90 gives you about an O85 circle.  An inner half circle of O81, O81, O72, O72, O72, O72, O81, O81 is about O76.  The two circles are concentric with the Atlas 4 1/2” spacing.  You can use other combinations of curve radii and as long as the curve section on the inner track is one size smaller (9” diameter) than the one on the outside track, the 4 1/2 track spacing is maintained.  For the equipment I run, 4 1/2” spacing on parallel curved tracks is too close.  I run all O72 on the inner track and the O81 with the O90 easements on the outer loop.  That gives a close approximation to Lionel O84 and O72.