The Peterborough & Shirley Railroad was incorporated in 1845, opening for public use from Groton Junction (now Ayer) to West Townsend in January 1848, and to the state line during the year 1850. In 1847, prior to its opening, the line was leased to the Fitchburg Railroad. The engineer responsible for laying out the line was George A. Parker (1822-1887). The Fitchburg purchased the railroad outright in 1860. Chief engineer of the Fitchburg Railroad when the present Nashua River bridge was constructed was Edmund K. Turner (1848-1915), widely respected as a railroad engineer.
On June 25, 1882 the wooden truss bridge over the Nashua River burned, and preparations were immediately begun to replace it with a "modern" steel structure. The pin-connected Pratt thru truss was completed in 1883, making it today the second oldest identified Pratt thru truss in the state (following Northampton's "Hotel Bridge"), and the only example of the form in the MBTA system.
Each truss, 128 feet in length, is composed of six panels. The top chords and verticals consist of two channels connected by lacing bars and a cover plate; the diagonals are 3-inch flat eye bars; counters consist of one-inch square tie rods; and the lower chords consist of two pairs of flat eye bars. The track is supported from the lower panel points by 24-inch plate girders.