Take a look at this: http://zhlw1.tumblr.com/image/146900969057
Please note the upward curvature (chord). I don't recall ever seeing it in a model. It is present also on unloaded flat bed trucks, empty flatcars, etc., but when loaded to capacity they will be arrow straight.
____________________ Junk is something you throw away three weeks before you need it.
Playing with Google's reverse image search found the history of that bridge, there were several before it. http://www.virginiadot.org/info/faq-covbridge5.asp
"The venerable Humpback Bridge lays claim to being the oldest of Virginia's remaining covered bridges. Located in Alleghany County, just west of Covington, it was built in 1857."
"Significance: The Humpback Covered Bridge was built for the James River & Kanawha Turnpike and is one of the early turnpikes through the Alleghany region of Virginia. It is the oldest bridge in Virginia, and the older of only two surviving examples of a cambered wood covered bridge in the United States."