Hi, initially we are not going to support Mental Ray in Max 2018. We found several technical issues related with the fact that now is an optional package, and not included by default in Max.
These are not easy to solve, and taking in account that many customers are planning to migrate to other renderers, we considere this issue a low priority.
Regarding Arnold, our intention was to support it. But unfortunately the integration with Max 2018 is very basic, and doesn't support well third-party plugins.
We have discussed this issue with Autodesk, and they are willing to make changes to solve that limitation. But by now, we only can wait and see.
Meanwhile, our advice is to use V-Ray, or one of the other renderers which are supported (you can find here
the full list, including supported features).
If you are in doubt with V-Ray, i encourage you to adopt it. It's a great render engine, and supports all Forest and RailClone features.