also 'ambiguity' is the correct word. Sorry, my english is VERY rusty by now.

the 2 last posts have been uploaded in the inverse order: the 1st was the 2nd, so I just corrected that mistake. . .

I think that, from a rigurous logic/semantic point of view, it keeps being a very ambiguous or slippery distinction, and we'll keep getting lost in the ambigüety of the use of concepts or, rather, in the words we use to express these.
the description 'a bridge that passes underneath a railway' can bring to one's mind more than just one picture. 'a bridge that supports a railway line (or railroad track) while crossing over a road or a river or something. . .' would probably evoke fewer pictures, or maybe just one with a bit of luck. If a word or sentence can evoke different pictures in your mind, or in different people's minds, well then it's not very rigurous from the logic/semantic point of view, but the English vocabulary -as well as every other language's vocabulary- has lots of terms that can be judged to (not) be so.
So, we could just conclude that everyone above has been right in their opinions from their own respective standpoints, and that, of course, it is very necessary to establish a fixed glossary of terms (no matter how semantically unsteady) to mean exact different things so to prevent unsafety (for example when handling trains or other vehicles, or machines, etc.) or confusion. So, be it as it may, this is a human society -that's to say, a very imperfect thing- but we just have to do what we can. . .

Just like my mistake: 'everybody have' instead of 'has'. . . for example.