Hi Joe,

How old is the kid? If he's younger than 8 or so, I wouldn't make the plan too complicated. I think the best for them is to be able to let the train(s) run around.
If he's a bit older, he might be interested in a point to point design, so the trains runs from A to B and back, with passengers or cargo.

You can then create a mine at one end, and a factory at the other to give the train a reason to go back and forth.

Do you already have any ideas yourself about setting and era?

David.