In North America there's a tendency to call anything a trestle that isn't an arch or a suspension bridge. Originally that seems to have applied only to timber bridges, but steel bridges - even long plate girders - are often loosely called trestles. The sketch in the opening post would be probably be described as a deck truss or an underslung truss by an engineer. The Bridge Basics web page is excellent, and gives the proper names for the specific types - it should be very helpful for modellers.
I was delighted to see the model based on the Connel Ferry Bridge - it has long been one of my favourites, and I always try to get a glimpse of it from the train on my trips to Oban (and just possibly, in the distant future, it may be a GME project).
Andrew