MPL Minnesota River Bridge

Located in Morton, this through truss bridge carries a former Minneapolis & St. Louis Railway branch line (now Minnesota Prairie Line) across the Minnesota River. The first bridge at this location likely consisted of a wooden truss bridge, constructed when the line was first built. In the late 1890s, the M&StL made several improvements to this line, replacing major bridges with steel and stone and replacing several smaller timber bridges with stone culverts. The present bridge was constructed in 1898. Throughout the 20th Century, the lengthy timber pile trestle approach on the west end had been shortened, and in 2024 the approach was rebuilt and strengthened, giving the bridge its present configuration. Currently, the bridge consists of a 157-foot, 6-panel, pin-connected Pratt through truss span, approached by an exceptionally long 87-span timber pile trestle approach on the west end. The bridge is set onto a stone east abutment, while the remainder of the structure is set onto timber pile substructures. Stone for the east abutment consists of granite, likely quarried locally in Morton. Phoenix Bridge Company fabricated the truss span, while the Widell Company constructed the abutment. The timber components were originally constructed by railroad company forces, and were rebuilt by E80 Plus Constructors.

The truss span follows a traditional 19th Century design, with light members, pinned connections and a standard floor system. Both the endposts and top chord use built-up beams, which are composed of channels connected by X-lacing on the bottom and solid plates on the top. The bottom chord uses eyebars, with one set of two at the outer panels and one set of three at the center panels. The vertical truss members consist of solid beams at the hip verticals and X-laced built up beams for the center members. All diagonal members are composed of eyebars, which are formed from rectangular steel bars. Thinner steel bars with turnbuckles form the counter diagonal members. The floor system uses standard I-beam stringers and plate girder floorbeams, which are placed at the panel points. The floorbeams are riveted to the vertical members, with the lower connections running through the bottom of the vertical members. Both the upper and lower lateral bracing are composed of solid bars, which form an "X" shape across each panel. The portal bracing uses a light A-frame design, constructed out of L-shaped bars. The sway bracing uses a V-laced beam, which is connected to the vertical members by a diagonal bar. The abutment is constructed of granite cobble, which is mortared. There are short non-standard wing walls on either side. The approach uses a standard design, which has been reinforced with new stringers.

Through the second half of the 19th Century, pin-connected Pratt truss spans were the most commonly used truss design for railroad bridges. The Pratt design was simple, durable and economical. By the turn of the 20th Century, riveted Warren designs had largely superseded pin-connected spans. Phoenix Bridge Company built a number of bridges for the M&StL between the late 1890s and early 1900s. Two additional trusses of this design were constructed at Spencer, Iowa and New Ulm, Minnesota; both of which have been removed. As part of the project that constructed this bridge, an unusual riveted Warren span was installed at the Redwood River nearby. It is unclear why the M&StL used both riveted and pin connected spans as part of the same project. Since the initial construction, the bridge has seen few alterations, except for general repairs and a significant rehabilitation of the approach spans. Today, the bridge remains occasionally used. Overall, the bridge appears to be in fair condition, with no significant deterioration seen on the truss. The author has ranked this bridge as being regionally significant, due to the unusual truss design.