It finally occurred to me to just make Simon a train track on the TV table where he loves to drive things back and forth. He was using a slinky as a tunnel, or driving his trains through the ferry boat, so it made me think that he just needed a tunnel. He'll play happily here for quite a long time, and it's always the first place he goes in the morning and right after school. In fact, he loves it so much he frequently cries when we make him stop playing trains to do useless things like eat breakfast or supper.
Whenever his trains come unconnected or his track falls apart, he'll come to us for help. I'm training him to go to Alice for help, and it's having a good effect on their relationship, as Alice learns it's fun to be a big sister since she can do more than Simon, and Simon can have interaction with Alice other than screaming when she takes his toys.
