Zoo Interchange Rail Bridge

Quadrangular Through Truss over Hank Aaron State Trail
West Allis, Milwaukee County, Wisconsin

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Name Zoo Interchange Rail Bridge
Built By Milwaukee, Sparta & Northwestern Railway
Contractor Pennsylvania Steel Company of Steelton, Pennsylvania
Currently Owned By Union Pacific Railroad
Length 170 Feet Total
Width 2 Tracks
Height Above Ground 25 Feet (Estimated)
Superstructure Type Quadrangular Through Truss
Substructure Type Concrete
Date Built 1910
Date Closed 2015
Traffic Count 00 Trains/Day (Bridge is Bypassed)
Current Status Bypassed and Closed
Former UP Bridge Number 89.81
Significance Moderate Significance
In 1910, the Milwaukee, Sparta & Northwestern Railroad would begin construction on a beltline around the west side of Milwaukee, to bypass the traffic in the core of Milwaukee.
At the time, the route to the Twin Cities was complicated for the Chicago & Northwestern, and involved going through Madison, Baraboo and La Crosse to Mankato.
During the same time, the Milwaukee, Sparta & Northwestern was working on a line which connected the northwest corner of Milwaukee to Wyeville, and ultimately Eau Claire, which provided a direct link.
The newly constructed beltway around West Allis provided a link to the new mainline from the current C&NW line around South Milwaukee.
When completed, the C&NW quickly took a controlling stake in the MS&NW, and absorbed the company.
The line was only 8 miles long, but one of the most significant in Wisconsin. The new construction included the massive Butler Yard on the far northwest side of Milwaukee.
North of the Butler Yard, the line would have an intersection with two other lines, built at the same time. One heading west, one heading east.
Dropping south through the Butler Yard, the line closely paralleled the Menomonee River and what is currently I-41 (Zoo Freeway).
Continuing, the line would cross the Milwaukee Road mainline between Saint Paul and Milwaukee, as well as Underwood Creek and a toll road towards Watertown, all on the same bridge.
Next, the line crossed what is now I-94, and crossed a secondary Milwaukee Road Line, which was the previous mainline.
The line eventually ended at a wye, where it met the line around the south side of Milwaukee, and the line towards Waukesha and Madison.

The line is known for being heavily engineered, including some of the technical marvels at the time, such as its trusses.

Today, with the acquisition of Chicago & Northwestern in 1995, the Union Pacific owns and operates the line as a fairly used mainline. While the bridge across the southern Milwaukee Road line was replaced in 2014, the line still retains much of its original integrity and components.

This bridge, which was built in 1910 to cross a line of the Chicago, Milwauke St. Paul & Pacific.
The bridge was one of the heaviest built quadrangular trusses at the time in the world, and set the stage for several years of a revived design of trusses.
Prior to the Milwaukee, Sparta & Northwestern constructing the variety of quadrangular through trusses across Wisconsin in 1910-1911, the design had fallen out of favor with several railroads.
The quadrangular through truss design originated as a lightweight truss design in the mid 1880s, built mostly in the upper midwest and distributed by Lassig Bridge & Iron Works of Chicago.
The design can actually be traced back to the lattice trusses Leighton Bridge & Iron Works of New York, which originally used the design on iron bridges for very early Chicago & Northwestern Lines in Minnesota and Wisconsin, creating deck and through truss varieties of the structure.
The three major structures were created in 1880. Crossing the Minnesota River at New Ulm, Minnesota; the Chippewa River at Eau Claire; and the Wisconsin River at Merrimac.
The design was popular on 1880s and 1890s Rock Island and Chicago & Northwestern lines, mainly due to its relatively simple to construct design.
As the railroads continued to expand, heavier types of riveted through trusses and girder spans appeared, reducing the need of many Quadrangular through trusses.

When the MS&NW teamed up with Pennsylvania Steel Company in 1910 to build these bridges, the design began a revival. American Bridge Company began taking on more projects for the C&NW which included an enhanced design, created with heavier riveted connections, and A or M frame portals, instead of traditional portals.
The bridge at the Zoo Interchange pioneered this, and is recognized as such.
In 2015, the bridge has been spared demolition according to one WisDOT employee. The bridge was bypassed in 2014 due to the new I-94/I-41/I-894 Zoo Interchange Project.
It is presently unknown if the bridge will ever be reused. However, the bridge has been spared demolition, and fortunately will be around for future years.

The author has ranked this bridge as being moderately significant, due to the truss design.
The photo above is looking from a pet hotel, during the construction of the new bridge.


Source Type


Build Date Pennsylvania Steel Company plaque
Contractor Pennsylvania Steel Company plaque
Railroad Line History Source ICC Valuation Information, Compiled by Richard S. Steele

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