If there is some sort of special event going on inside the Olympic Park, Line 8 may be restricted access to ticket or pass holders. However, special events being held there are rare. Also, Line 8 is only a minor spur route right now serving the Olympic Green and Forest Park, so if the Powers That Be want to, they can shut it without inconveniencing most of the population here. Some things you can't predict, this is China. In the unlikely event Line 8 is shut during your visit, you can walk or taxi the balance of the distance.