Rory – Here’s an idea I picked up from somewhere you might consider. I think it’s called the “spline” method. (Sorry for the crude drawing, I used Paint, and it’s definitely not to scale.)
Anyway, Fig. 1. represents a grade starting at point A, continuing around a curve that starts/ends at points B/C, and then levels off at point D after rising a total of 2”.
Fig. 2. is the profile of the grade – picture a side view the whole thing straightened out, with where the curve started and ended marked. Fig. 3. is a top view of the same thing. You can include the transition to/from level to grade in the profile, by the way.
What you do is take a piece of foam THICK enough to secure your roadbed to its EDGE and cut it into the shape of the profile. Then, anywhere along it that would be a curve, cut it into segments maybe 1-2” wide as shown in Fig. 2. Probably you should label these parts to keep them in the right order.
Then glue the pieces onto your layout. Locate the parts that go under curves so they touch adjoining pieces only at the corners that would be on the inside of the curve. The corners that would be on the outside of the curve are spaced apart so that the pieces follow the curve. Then fasten the roadbed to the top of the whole mess.
One problem I can see is cutting the foam so that the rails will be level with each other and there’s no unwanted dips or rises. I’d probably use a power jigsaw or whatever to keep the cuts straight.