Audio books, then you can cuddle up together and follow the story. Relaxing for you and hopefully him.
I would try to engage with the trains, I totally understand where you are coming from, finding them boring. I had the same issues with my nt 8 yr old when he was little... He loved little people toys, I liked to set them up with him but the playing drove me a bit nuts. He so loved it though when I did play with them, with him. Very special for him. If you could do 10 mins with your son and see if he adores it, don't be afraid to set a time limit, tell him it's not your favourite thing to play but you know he loves it, so you'll give it a try. You never know, he may not want your involvement. Train tracks for my 5 yr old ( pddnos) are a solitary activity, he thinks he wants others to play but we always do it "wrong", so now he chooses to play them by himself. He loves this other game where you have to make frogs jump in to a bucket, a bit boring but we always have a laugh, it lasts less than 10 mins and gives him some good, fun time with mum or dad. It's low stress, not much can go wrong in that one.
Also colouring in or drawing together.
Lego, big hit in our house too, it helps I can build faster than the kids, so I can duck away in the midst of it to do jobs and not miss out.