Structures like this one used to be quite common in Wisconsin. They were built by the railroads, I suspect they were cheap to build since the structures used materials that railroads used to build river and road trestles for the tracks.
As a kid, I remember the cheap thrill of being in the back seat of The Old Man's '58 Ford when he'd hit the wooden approach with a smack, listening to the rumble of the tires on the wooden planks, then the feeling of flying as the car bounced up on the deck, and flew off the deck on the descending approach span on the other side.
There were two of them, only a couple hundred feet apart, on Teutonia Avenue in Milwaukee, that carried Teutonia Avenue across two competing rail lines. (They were replaced by one long UCEB, there's a photo taken from it at http://bridgehunter.com/wi/milwaukee/bh38477/)
It felt like we were flying when we hit those bridges. Of course, at 35 MPH in a heavy old Ford, we weren't even close to flying, but to a young imagination, it was a thrill (if not a little terrifying) to cross those spans.
Except for a few of them on rural roads, there aren't many of these wooden structures left anymore.
This listing contains an excellent set of photos showing a wood beam truss. Very enlightening.