My first impression was that this bridge was very tall. It has about a 330 ft span and it's about 30 ft tall, so maybe this kind of truss requires a 1 to 10 ratio between its depth and its span. I would guess that its a Pratt Truss with all the diagonals in tension, but I'm not an expert on truss bridges. If you know what type of truss this is, please let us know.
Truss bridges must have seemed the height of scientific progress in the 19th century, but now they are seldom used. Before computers, it was relatively easy to determine the forces on these structures that have no flexural members, just compression and tension members with pinned connections. They still seem to be a very efficient method of spanning long distances with a minimum of steel. Maybe some innovation in truss bridge design will bring them back into fashion or maybe they'll eventually disappear.
American River Bridges: Folsom Historic Truss Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.