Props (trees, rocks, buildings etc) cannot be modded into the game yet, this feature is still to come. You can mod trains though.
There is a guide on Steam for how to do this:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1789707835

Thanks - that's too bad... I'm mostly interested in creating objects. Will check back.

You can make a rock object and call it a train, but the rock on a track and lower it. that's my idea...

put*

you can create scenery and place it on your map as a quickTerrain but it has it's limitations and difficulties. Best used for large and complex infrastructure like bridge and station buildings.