IMO, it would be good if FPP would automate that setting if it detects you've chosen an RC object as surface. Is that possible or easy to do, lol?
I'm afraid it's not so easy. Forest requires a valid mesh as surface, because it needs to compute many values as hitting points, normal vectors, etc.
But in most of cases RailClone doesn't generate a complete mesh, but a collection of render instances (similar to proxies). These elements have not a "real" geometry which Forest may query.
Futhermore, in V-Ray the segments that are not instances (bended pieces, sliced, etc.) are built at render time and only if needed, when the raytracer hits their bucket.
For these reasons it would be very complex to use RailClone as surface without forcing it to build a mesh, as described in previous posts.