Bab has it...several of us have used camera tripods and wrapped a quality twine around the mounting screw. I prefer fishing line, something with little give such as Fire line. Then with pencil in hand, pinch the twine to the pencil where you want the centreline and draw your arc. I have also used a trammel by placing a nail in the pivoting end and inserting the nail tip into the hole where the panhead screw rests. The trammel is pre-drilled with holes large enough for a pen tip to protude below the nether surface every inch to a suitable range...labeled for its corresponding radius.
Still another way is to fix a steel rule on its side, or maybe a strip of wooden lath, and bend it to conform to a reasonable centreline...you'll also get your easement this way. Drive nails into the subroadbed to keep the one end of the steel ruler or lath tightly secured along the centreline, on its side, and along the last bit of tangent before the curve. Then, bend the free end and draw the resultant eased curve. You should be able to trammel the rest of the curve, until you have to reverse the above procedure to get the easement out of the other side of the curve.
I have heard of people using camera tripods to act as the pivot point for the trammel. I have used a piece of 1 x limber with the point of a nail sticking up through the flat surface at one end of it to act as the pivot point for the trammel. I clamped the other end of the 1 x to my bench work. I then attached my trammel to the nail point and made final adjustments to get the pivot point in the proper positon to draw the desired curve on the subroadbed. This may take a little time to get it exactly right.
Hope this helps,
Bob
Your cork will have squared off ends as an artefact of the milling process. When you lay the sections on a curve, you will necessarily have ends that don't want to meet at their outer ends...along the outside of the curve since two 90 deg surfaces can't be in contact on a curve. So, you could impart a slight angle to the ends with a sharp knife, or file or sand them, so that the ends meet better. This will in turn make planing or sanding the tops of the cork sections a bit easier.
Also, the roadbed is just that. It needn't conform to the easements to quite the extent that the tracks above them should. Still, use the centreline you ought to draw for your eased section, and lay a thin covering of latex caulk along each side of the line. Then, place your cork, one piece at a time, and use push pins every 3-6" for securing it in conformity to your centreline. Add a bit of weight (a soup tin every few inches) and then wait for about two hours. When you remove the pins and soup tins, you should be quite happy with the utility of your work. Don't forget to sand the surfaces of the roadbed cork to ensure smooth track, particularly at joints.