It is complicated, not surprised when you are dealing with the law and the lawyers who create them. There are some solid things you can assume, anything published before 1924 is public domain. For things published between 1924 and 1963, it is possible it is in the public domain because they in those years, you had the option to renew a copyright, if they didn't renew the copyright then the material would be public domain. For example, the post war Lionel manuals created before 1964 could be public domain now if they didn't renew the copyright on them. For works created after 1964 it likely is under copyright, it is user's life+70.
Then, too, there are copyrights where the holder doesn't bother to enforce it, there have been court cases where the copyright holder tried to sue for infringement and lost on those grounds(Xerox tried that with Apple Computer, who took things like the mouse, hypertext and the graphical user interface from Xerox, but because Xerox didn't try to defend their copyright until many years after the fact, court ruled they had failed to defend it and de facto invalidated it).Copyright holders can also place their works in the public domain explicitly.
Basically, what I would argue is unless you know specifically something is in the public domain, that you get permission for publishing it. While in practical reality someone who scans in the wiring diagram from an old lionel manual and posts it here may not cause any issues the folks running the forum could potentially get a nasty letter or phone call from someone, you just don't know, especially if you are scanning in something from a currently under copyright book (I suspect that if they are using for example schematics from a post war manual, they likely got permission to use them from the copyright holder).
As far as fair use goes, there are pretty strict rules around that, it generally comes down to use in teaching , for the purposes of parody, criticism or commentary,plus it is also covered in legal proceedings/writings, where for example in a write up of a court case writings from a book were part of the evidence in the case. . Thus in a book review, the reviewer can use snippets of the book in the review, since it falls under criticism...and that is pretty much it. you could argue that a non profit generating thing, like this forum, is about education, but it likely would not pass muster (and given that OGRR would face the infringement, I doubt they have the time or resources to hire high priced attorneys to allow publishing a wiring diagram from an F3 from 1955).
It is why it is much better to publish links to an outside website with the information, either they have gotten permission to have the material or if publishing it illegally, that is their concern
