I've always rather liked the rivetted plate bridges which are commonplace all over the railway system and which are the subject of this post.  I've included a couple of GWR bridges in the Bristol area for reference and from which the particular shape of section supporting the raised steel parapet is drawn (Google "fair use" credit): it may be particular to GWR plate bridges as I haven't noticed it elsewhere. For general reference on steelwork (and in particular on rivetted structural sections) Dorman Long's 1906 handbook is useful (https://www.dormanlongtechnology.com/Download_files/DL%20Historical/Dorman%20Long%201906%20handbook.pdf).  It covers compound girders such as the one I've represented at SwanHill together with things like rivetted columns and column cappings as well as standard equal and unequal angles, Tees and so on.  There are some illustrations of bridges at and after page136 including cross sections of single and double track railway.  The photos show the build up of the compound girder with a raised parapet, both about a scale 6' high over a span of 65' (so a span to depth ratio of about 1/11).  The bottom flange will carry a coffered brick soffit running back under the railway to an equally long beam which is supported mid span, so less deep and buried underneath somewhere.  It may seem counter intuitive for the more detailed part of this structure to be facing away from the "front" of the layout but the beam soffit can be seen from underneath and it may be that "the railway" is not always
housed where it is now. It's my intention, where there is a front and a back to something like this girder, it will have both front and back, whether seen or not. 

There are a few rivets on this model - I haven't counted them - put in with a handy little bench top rivet embosser, a pillar drill ex Ebay "for spares" re-purposed as a staking tool and rivet press.