People should also have a registered version of Trainz before they can post anything. That would single out many people as well.
That has been suggested but, unfortunately, one of the most common posts made by new unregistered Trainz users is "how do I register?". A better suggestion might be to allow them limited posting rights in something like a "new users" forum until they register. There they should be able to ask basic "newbie type questions" but I suspect that most of the "old Trainzers/Geezers", who might be best at answering those questions, might not be inclined to venture there - I could of course be wrong!