Thanks for contacting us. This problem occurs because both Forest Pack and RailClone don't support Object IDs or other channels that depend of the item's ID. In V-Ray Forest is handled as an unique object with its own ID, and it's not possible inherit or assign multiple IDs. However you can use material IDs to get the same effect. To do this:
- Add a VRayMtlID render pass
- Choose Integer
mode for no AA, or Colour
mode for AA.
- For the glass material, assign an effects ID by selecting the material and assigning an ID greater than 0
- If you want to assign additional IDs and you need more than 16 you can use the V-Ray material's Override Material Effect
There's also a nice new script by David Wortley that can automate this process called Power Mask
That's it. I hope that helps. Please let me know if there's anything else I can do.