Charlie and Rader are both correct.
Part of the reason why this bridge was saved by (and rebuilt through the efforts of) the National Society for the Preservation of Covered Bridges is that it is such a rare example of what was once a very common design. In fact, it was the preferred bridge design of the Boston and Maine (mostly NNG) railroad - with dozens of this design either on the railroad, or road bridges that spanned the railroad. While the historic WW&F did not employ this particular design, two different road bridges did; they spanned the Sheepscot river in Alna very near the Head Tide Station.
Today only six of the "boxed pony" design exist in all of North America. We're happy that we were selected to be the caretakers of such a unique structure.
Ed Lecuyer
Trainmaster, WW&F Railway Museum
Help the WW&F Build Locomotive #11: [
build11.wwfry.org]