Anything to keep the drama going. If it is your son's train, then he should decide where he wants it to be. If your step-father purchased the train for your son, then it is your son's train. I think I would have a chat w/your step-father about the issue and see what he would advise...maybe he'll purchase your son another one for your house.
I am so sorry that your xh is at it again, but he's really trying to keep you and your kids sucked into the drama.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.