This is a former Chicago, Burlington, and Quincy railroad bridge over the Illinois River, in the town of Ottawa. It's a fairly rare lift-type bridge-the center part raises up to let river traffic go through. The first bridge at this location dates to 1868, but I believe this one was built in the 1930's, when the lift span was installed.
I've seen this bridge several times on my travels to and around Ottawa, but had never managed to get a good shot of it. It's a good looking example of an older iron rail bridge, and the Illinois River valley in this area is pretty picturesque. But I got lucky this weekend, as I was passing through on my way home, a little later than usual, with the winter sun low in the sky.