Maintaining rail service quickly became the project’s defining constraint. The Milwaukee District North Line carries Metra commuter trains, Amtrak service, and freight traffic, making extended shutdowns impractical. At street level, Milwaukee Avenue also had to remain open as a busy arterial roadway.

This dual requirement, keeping both rail and roadway systems operational, shifted the focus from simply replacing a bridge to designing a construction approach that could support uninterrupted service. Early concepts such as staged replacement and temporary single-track operations were evaluated but proved difficult given the new bridge profile and configuration.

The selected solution was to construct a temporary two-track shoofly east of the existing alignment, allowing trains to continue operating while the new bridge was built.

Although temporary track is common in rail construction, fitting two operating tracks into this corridor required careful coordination. The alignment was bounded by retaining walls, adjacent properties, a maintenance access road, and other rail infrastructure. Constraints at both ends of the project, including Grayland Interlocking to the north and existing bridges to the south, limited how far the alignment could extend.

Designing the shoofly involved more than shifting track laterally. The temporary alignment had to accommodate reverse curves, maintain acceptable operating speeds, and provide space for temporary station platforms, all while creating a safe work zone for construction. The alignment effectively had to “thread the needle” through an already crowded corridor, requiring close coordination among bridge, track, and civil engineering disciplines.

Given the complexity of the site, traditional plan sheets alone were insufficient. The team used three-dimensional modeling to evaluate how temporary alignment, retaining walls, embankments, and the permanent bridge would interact throughout each stage of construction.

By modeling the full corridor rather than individual elements, the design team was able to better understand spatial conflicts and coordinate across disciplines. This was especially important where geometry became more complex due to the skewed intersection between the railroad and Milwaukee Avenue.

This approach allowed potential issues to be identified and resolved during design, improving coordination and reducing uncertainty before construction began.

The structural design introduced another layer of complexity. The replacement bridge is a continuous through-plate girder structure with long spans and significant skew, resulting in more complex load paths than a typical railroad bridge.

To better understand how the structure would perform, the design team developed a detailed finite element model. This analysis allowed engineers to evaluate behavior under loading conditions, refine member sizing, and confirm constructability prior to final design.

Rather than serving only as a visualization tool, modeling became an integral part of the engineering process, supporting more informed decision-making and helping both designers and contractors understand how the project would be built.

Beyond the bridge itself, the project highlights how construction challenges can shape better long-term solutions. The design team treated the project as a collaborative effort across multiple disciplines, where decisions about structural design, staging, and temporary works were closely integrated.

One example was the approach to temporary retaining walls and support systems. Instead of viewing them as disposable, the team identified opportunities to incorporate select elements into the permanent design. This reduced material waste and streamlined later construction phases.

The project also demonstrates the value of integrating modeling and construction planning early in the design process. On a site where available space was highly constrained, understanding how each element interacted over time helped improve coordination and reduce risk during construction.

Bridge A32 and Grayland Station now provide safer, more reliable service along the Milwaukee District North Line. Just as important, the project demonstrates how complex infrastructure can be delivered without interrupting critical transportation operations.

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