Joe, as Bob said, use trapezoids to make up the curves. The zoids are 1/2 inch birch ply ripped to 12 inch width and then cut on a 15 degree angle. Length at the midpoint or center is 18 inches. Upper curve is 088 lower curve is 072. Center rail to rail separation is 6 inches. I needed 2 1/2 inches from the outer rail to the wall for the overhang on my BigBoy. Zoids are joined using 1/2 splines - you can see the screw placement at the joints.
I guessed at the 18 inch length. If I redo these, I would probably cut 14 inch wide boards, but keep the top length at 21 1/2 to give the extra 2 inches at the bottom. If we calculate the perimeter length of these 2 curves, we should have center rail arc lengths of 69 and 56.5 inches. Center length of the zoids roughly total 72 inches.
I have to lay out the zoids for a single track curve with a width no more than 7 inches. Thinking about how to do that is giving me a headache. While the total arc length stays constant, each zoid will have to be significantly shorter with a corresponding shallower angle. The issue is to minimize the variation between the corner joint and the center of the zoid. I think I'll need 8 sections, maybe more, so that the track will nominally have the same amount of wood on either side.
