The 36" Super-Flex Track (Atlas H0 Code 100 article number 168) has a length of 36" so at the default 45º arc angle - that is applied when first bending a flex track to a parametric curve - the maximum radius is 45.8" before exceeding the track length.
The app autocorrects entries to ensure flex tracks in the layout reflect what's physically possible.
When reducing the arc angle - e.g. to 20 degrees - a larger radius can be used;
The next maintenance update shows a message in the status bar at the bottom of the window to indicate why a radius or angle was rejected.
Hope this helps,