It seems this topic comes up just as we have forgotten about the subject.
Sell your trains at the last possible moment. In other words, enjoy them for as long as you are coherent, then start parting with them.
My grandchildren are still young. I set up a small layout for my oldest grandson a couple of years ago. He has some interest in it, but so far I don't see it going anywhere.
As others have said, once you're gone, you are gone. Unless there is life after death, there's nothing you can do about what happens to your trains or any hobby items.