To quote from: "Historic Highway Bridges of California" 1990, California Dept. of Transportation: "The Purdon Bridge is the only known example in California of a half-through truss, an unusual truss form in which the deck is carried approximately half-height between the top and bottom chords... The bridge was designed and built by Cotton Bros. and Company.
Very unique! Have never heard of a "half" through truss
The bridge is open and can be crossed by vehicle. It is a single lane bridge.