Thanks for the reply. Yes, I noticed that I could not getting ANY railclone object to render after upgrading Arnold.
But I'm still stuck. I have not found a way to downgrade the Arnold version in 3ds Max 2022. When I try to remove it in Control Panel, it appears to work, but then if I try to install and older version of Arnold, it says that Arnold is already installed. Sure enough, when I reboot, the latest version of MaxtoA is back in the Control Panel again. It appears I can't remove it.