This issue happens periodically, sometimes it works and sometimes it doesn't. Using the PMDG 777F and the cargo train refuses to move on both deboarding and boarding. So when deboarding the train doesnt move from it's position and requesting boarding would spawn a cargo train with cargo loaded but also refuses to move.
EDIT: So I just tried again, and it turns out it only starts to move after the other cargo has been offloaded
