Just for future reference, in O-Scale, the "number" of the curve represents the rough diameter in inches , so an O-72 curve is about 72 inches in diameter and obviously would only fit inside a table top of slightly more than 72 inches wide.
I use terms like rough and slightly because it depends which track you are using and whether the diameter is being measured from centerline to centerline of the track or not. Plus, you generally want to leave some space between the outside edge of the track and the layout top edge, especially if you install edging.
So, in the above example of an O-42 recommendation, if that distance is measured from the centerline of the track (the middle rail) and it's 1.5 " from the centerline to the outside of the ties or molded roadbed, you're now up to 45" for the total width. Add another 1.5" on both sides for clearance and you're right at 48".
If you happen to be using Fastrack, an O-31 curve will not fit inside an O-36 curve unless you add an extension piece or pieces to the O-36 curve to spread it out.
Hope this helps.
