Washington Street Bridge

Iron bridges spanned the era between wood and steel. Those like the one on Washington Street are sometimes called “Pumpkin Seed” or “Bow String” bridges and were built throughout New England and beyond by the Berlin Iron Bridge Company. Officially known as lenticular truss bridges, they follow principles of a patented, “parabolic, lenticular, truss design employing paired elliptical arches connected at the ends and cross-braced between. The trusses which carried the loads used cast iron for compression members and wrought iron for tension. Berlin Iron and Bridge would build nearly 400 lenticular truss bridges throughout New England, NY, NJ, PA and as far away as Texas. The Washington Street bridge, built in 1879 when Berlin Bridge Company was still called the Corrugated Metal Company, is thought to be the earliest of them.

Interesting info here: http://next.owlapps.net/owlapps_apps/article?id=67923819&lang=en