Bridge type
One of the things described in Mallery's Book, is an engineering principle called "Cooper Loading".
IN essence, the length, weight of average tonnage using the bridge, and the availability of material can dictate these design principles. It also might be determined by what it is you are spanning, i.e., roadway, creek, river, etc. A railroad also might determine that for a slow flowing creek, a culvert might do, in that when original surveys were done, the railroad always built with an eye towards cost. If a fancy bridge wasn't necessary, it didn't get built, and something cheaper may do until that design failed (flooding or other disaster).
The books go into that in detail.
There is also a book available on American Railway Bridges (I don't remember the publisher).
chapmon