Thanks to all for the input and to Dean for the pictures of that great layout. My new plan borrows from all responses. I will sawcut a slice from the sidewalk, install a concrete sub-base, fill to grade with PT or composite board material. Then, I can route a passage for the rails in the wood or composite. I saw something similar in Garden Railways on a wood deck. Rails can be siliconed, epoxied or screwed down for stability. If I were crossing perpendicular to the sidwalk, I would ballast in the area just like Dean. However, I anticipate that I may be crossing at an angle, perhaps up to 45 degrees, making solid, fill-in material necessary.
Dave