Among my three grandchildren, one of them loves playing endlessly with Brio, one enjoys them for about 10 minutes at a stretch and the third just beats the shit out of them.
The one who really loves them prefers to have me down on my knees with him the whole time, but that is a delight. He’ll be four next week and I’m just about to introduce him to electric trains.
Every kid is different, and they change. A local pre-school or library might take the Brio if your family isn’t interested.
I’m not sure what happened to my post.
But I was in essence saying as soon as Grampa makes arbitrary rules about playing with the toy the game is over.
Always.
Of course as soon as the kid starts throwing toys the game is over for a different reason. Always.
Give the toy to an age appropriate day care center. Walk away with a bittersweet feeling in your heart.
All things must pass.
Pretty accurate assessment. 
Donate it to a place that takes care of kids.
They might not have space. Retirement homes may also be an option.
My wife and I just moved to a home twice the size or our previous home. We donated 14 SUV loads of stuff (yes I’m missing some stuff that got accidentally donated)
I’m getting Sheldon vibes.
They were having fun wrong.
I don’t know, maybe I’m wrong, but I’m sensing a disappointment in the grandkids that goes a little deeper than “they’re a little rough with the toy trains”. OP, is this just a obvious issue of deeper concerns you have with the kids?
I never would have thought of this as an option, but I think it’s brilliant. There’s absolutely no reason retirees can’t have fun, too, and it may be therapeutic for them to boot.
Ask your son, then donate when he says no.
My sister’s grandkids think she has all the best toys. She has lots of Lego, she has Rokenbok. She has Tonka. She has Manga-tiles. And she’s 1700 miles away, so they only get to play with her stuff once a year.
StG
I’d Have You Anytime