I would suggest that the term: "Bridges" is far to vague to search for.
Please provide a Google Images, image of the particular bridge that you are looking for, and I will help you look.
Most backwoods NG lines had limited resources and money, so rough timber structures were usually employed, rather than expensive extravagant steel bridges (there are many exceptions to this).
Also a SG, or 40", 36", 30", 18" bridge, can be slightly lower and hidden underneath your 42" NG track