I screwed a board onto the benchwork, using a single screw, so that it extended out into the aisle, then positioned my radius tool (a long stick with holes drilled in it at set intervals) so that it seemed to be in the approximately correct area - the single screw allows to board to pivot so that you can reposition the pivot point for the radius tool. Then clamp the board in place while you try different radii for the location. Of course, this only works for inside curves: You could use a cardboard template for outside curves, and since you'd have to make it for the outside curve, be just as well-off using it for the inside ones, too.
:-D
To be quite honest, I sawed up a couple of sheets of 3/4" plywood into expanding curves, starting at my minimum of 30", and working up to, in 2" increments, 48". The curves were laid out starting at the narrow end of the sheet, and the majority were in the 32" to 38" range, as these sizes seemed like they would be the most useful for my particular situation. With an armload of curved roadbed, I just walked around the room plunking down the widest ones that would fit in each particular location, and with a room like the one shown below, you can bet that there are a lot of curves. Sorry, but I don't have a trackplan.
Wayne