This should be possible but there are just a couple of settings you may need to change.
- Using RailClone as a Surface in Forest Pack disables RailClone's instancing capabilities. This is only a problem if you're generating huge poly counts but in this case it seems like most of the high poly detail is beging done by Forest Pack so you should be ok. I often disable the instancing engine from RC's Display rollout, just to avoid any problems though it should happen automatically
- Don't forget that for Forest Pack to update after changes to RC you'll need to update the surfaces. You can turn on Auto to avoid having to do this manually each time you make a change.
- Finally, make sure that RailClone's Display mode is set to Mesh
. If it's displaying boxes the Material IDs are not set correctly, which could be the problem in this case.
That should be all you need to use RC as a surface in FP. I hope that helps, please let me know if you have any further questions.