Hey OP, I had a similar problem last week. I was using multiple xrefs as well, that contained RailClone and Forest objects. After going though each of the xrefs, I discovered none of them had any issue rendering on their own.
It really just came down to some sort of mystery corruption in the final render file that just had cameras and an HDRI in it. Starting from a blank file, importing everything from the corrupt scene (even the render preset), solved the problem for me.