Well, that's easy, try this:
1. Connect storage boxes to dockers on both entities (Using C and V like in all connectable blocks, Docker on ship, Basic rail on station).
2. On the source entity, turn off storage pull (If both pulls are active nothing happens).
3. This can be reversed easily by just controlling which storage is pulling.
4. Make sure you have enough storage for cargo ops, and understand how to set up pulls.