Metra Z-100 Bridge Replacement

Gannett Fleming teamed with prime consultant TranSystems Corporation to develop design documents for a new, two-track structure to replace the existing Metra Z-100 bridge that is now controlled by a modern, PTC-compliant signal system. The project site was comprised of complicated geometry, including:

• A bend in the river.
• The overhead U.S. Route 20 bridge.
• Overhead power lines.
• Proximity to an existing Union Pacific Railroad bridge.

Gannett Fleming’s primary tasks within the design team were the overall bridge concept, superstructure design, and hydraulic study.

Four 9-foot-deep girders were used in the first stage, and five more identical girders were added in the second stage. The design allowed for tracks to be in any configuration transversely across the bridge width, necessitating two independent structures. This design feature required careful consideration of construction staging and included temporary ballast retainers. The steel design included unique details to accommodate the bridge’s 30-degree skew.

The new bridge now has a ballasted deck, providing a superior ride, less expensive maintenance, and better protection from moisture and salt damage than the existing open-deck design. The existing stone masonry piers were rebuilt using concrete, an economical alternative that provides similar strength and greater resistance to longitudinal forces. With two fewer piers than the existing bridge, the new three-pier bridge reduces obstruction to water flow in the river below.

This project replaces all signal components near the bridge, including the wayside signal devices, highway crossing system, switch machines, snow melters, and backup generators. A new interlocking was installed, sheltered in two new relay houses on either side of the bridge. This modern equipment is more automated and compliant with PTC standards. With solid-state systems rather than moving parts, the new electronics require less maintenance and experience fewer breakdowns. The new backup generator and snow melters will help ensure reliability in inclement weather.

Double-tracking the bridge removes the delay-causing bottleneck at the Fox River Bridge. Train operations are less vulnerable to blockages on the bridge, and the second track makes it easier to schedule train meets, adding flexibility to increase future service.

• Double tracked spans supported by two cast in place abutments and three cast in place piers.
• New highway crossing system, switch machines, snow melters, and backup generators.
• New interlockings in relay houses on either side of the bridge.

• Eliminates train speed restrictions.
• Improves reliability and operational flexibility.
• Reduces maintenance costs.
• Ensures efficient operation of the Chicago region’s rail network.
• Saves 36,000 passenger hours to Metra rider travel times annually.