I'm checking this, and as your pointed out, it seems to work in some cases with FP 6.2.0.
But this was non intentional. Perhaps Forest was taking the RailClone viewport mesh, or other cached data. It's difficult to know, without checking all code changes between FP 6.2 and 6.3.
Really, it's not possible to use RailClone as Surface when Instancing is enabled, because RailClone doesn't generate a real geometry. Segments exists only at render time.
The only workaround is to create a separated RailClone object, used exclusively as surface, and hide it from render. You also should simplify this secondary RC, to avoid generating a huge mesh.