A long time ago, I think I mentioned an old bridge my grandfather, a civil engineer, built in the 1880s in southern Colorado. It was recently restored and rebuilt and is now open to traffic. It is a single-lane, wrought-iron truss bridge in a remote area, and the materials were hauled in by horses and wagons. There are only three known to exist anywhere in the country now, and it is listed on the National Register of Historic Sites.
Here is a newspaper article with a photo:http://www.chieftain.com/metro/1148731764/16
More info at (part-way down the page -- look for the photo):http://www.slvdweller.com/archives/2006_05.html
This article has links to earlier articles too.
After 125 years, it's essentially as good as new. The extremely dry climate doesn't cause a lot of rust and metal deterioration.