The simple-span Pratt thru truss is one of the oldest known examples of its type in the state, predated only by a highway bridge in Northampton and an 1883 railroad bridge on the Ayer-Groton line. It is tied with two other P&W Pratt truss bridges over the Blackstone River: a very similar thru truss in Uxbridge, and a 143-foot long deck truss in the town of Blackstone, both constructed at the same time. The Blackstone deck truss was built by the Edge Moor Bridge Works; the other thru truss in Uxbridge, 123 feet in length , was built by the Pencoyd Iron Works, like the Millbury example.
The bridge is composed of five full pin-connected panels , each 18 feet in length. Because of the bridge's skew, only two (opposite) corners of the bridge have inclined end posts, which, like the top chords, consist of paralled channels connected by cover plates and lacing bars. Vertical posts and bottom chords consist of channels connected by lacing bars on both sides. Each diagonal system consists of four eyebars; the opposing counters consist of two eyebars each. The cross beams linking the panel points of opposite trusses are 44-inch deep plate girders with a 16-inch web. These support six longitudinal beams, each a 22-inch deep plate girder, upon which the track ties are laid.
The Providence & Worcester Railroad was designed to replace the old Blackstone Canal, and in large part it followed closely the route of the canal. Completed in 1847, the railroad initially crossed the river here on a wooden Howe truss, replaced in 1889 with the present structure. John Waldo Ellis (1845-1916) was chief engineer of the railroad from 1869 until its absorbtion [sic] by the NY, Providence & Boston in 1888.
After the Bussey Bridge disaster of March 14, 1887, the state Board of Railroad Commissioners made a thorough inspection of all railroad lines and bridges in the state. The result was a meeting in November 1887 between the Board, and the P&W's Superintendent Charles Howard and engineer Ellis. The state Board's engineer George Swain stressed "the urgent necessity of immediate attention" to several of its bridges. The three 1889 Pratt trusses appear to have been the immediate result o f this meeting. By 1890, the railroad had replaced or strengthened near 40% of its 36 bridges, and six more pin-connected iron trusses had been erected.
Although the bridge was not completed until 1889, Ellis may have been responsible for the design of this bridge, begun the last year of his tenure. Under Ellis's direction, the road had been double tracked, branch lines constructed, and bridges, stations, and other structures rebuilt. A Woonsocket resident, Ellis also served as the town engineer for Blackstone.