In what way do your AF cars have a problem with 42" diameter? Could it be a matter of putting a drop oil on the various pivot points on the trucks or loosening the attachment post to allow a little bit of play? Or is it a matter of the body swinging too far out on curves (which can only be fixed by using a wider radius)? Train too long so that it pulls the cars to the inside on curves?
AFAIK, the reproductions are nearly identical to the originals - did AF use the same radius track as Lionel back in the day?
I have both 42" and 72" diameter. Can't recall any problems, but don't remember if I've run my AF cars on the 42" or not.