I've driven over this bridge more than once in my life and crossed it again today as I brought my cat to the vet in Newburyport. This is a swing drawbridge at the third span from the left. That alone is unique given its shape, but it's the combination of various truss types such as the Pratt pony, Warren, and Pennsylvania Truss bridges, which makes this even neater. The bridge has been recently updated so it can withstand a few more decades, at least of road traffic. It was originally built in 1883 by the Boston Bridge Works and updated over time, and has some historical significance due to its various bridge structures.
http://historicbridges.org/bridges/browser/?bridgebrowser=massachusetts/rocksvillage/
http://historicbridges.org/bridges/browser/?bridgebrowser=massachusetts/rocksvillage/