Let me guess - is this train some sort of collector's item that has resale value? Or is your ex just nutty about trains and freaked out that he could no longer use it himself?
Did your son want it at your house?
I can't imagine that the police told you that you legally have to return it. Seems to me that if it came from your family, and belongs to the child, the child has the say which house it lives in. Is there really any standard to declare the kids' TOYS as property to be divided in the divorce??? Is it mentioned at all in the divorce papers??/
And yes, you ex is a gigantic weenie for putting your son through this. Good riddance.