Bridges are my favorite structures, I remember wanting to model this one particular truss bridge my dad use to take me and my younger brother to, it was abandoned but it crossed the Conrail Cleveland Pittsburg line, Conrail removed it before I could take dimensions (darn double stacks
) so I searched for photos ect and then went and measured the abutment and hence got the centers between trusses so since I worked in CAD (I was entry level in the engineering dept) I used some of my down time (which sometimes could be entire days to research and draw this bridge up, kinda like a bridge forensics
I laugh thinking about it now because the firm I worked for at the time used to call us new guys "CAD punks" well this Cad punk got paid to draw railroad bridges
(where did those kinda work days go
) . But anyway after that bridge was removed I went and measured quite a few railroad structures like 5 bridges and a light tower an old B&O warehouse station ect and all are torn down now except I think 2 or 3 of the bridges but I got them on CAD! Now if I can only finish building the models I'm working on. But with that here is some more archived info this has Pennsy bridge clearances along with many other RR bridge specs. Again yes most is useless info for N scale purposes but if you skim it you find some useful modeling information, unless you want to calculate some N scale shear
http://books.google.com/books?id=9SI5AAAAMAAJ&printsec=frontcover&dq=web+member+angle+to+horizontal+on+curved+chord+truss+bridges&source=bl&ots=RUspJ9oYhU&sig=LoJafB5__skYR6cl7Yt67uEcCnY&hl=en&sa=X&ei=VP47UMuAPfH46QHYhYGYDQ&ved=0CFoQ6AEwCA#v=onepage&q&f=false