The bridge configuration consisted of a long wooden trestle on each end. The center of the bridge had a 485 foot long steel truss swing span, thought to be the longest swing span in the world when it was built. A 325 foot humpback steel truss was on each side of the swing span. The main path though the truss spans contained 2 parallel railroad tracks. A wagon bridge was hung off of the western side of the structure. Since the rail line required a gradual slope, the wooden trestle extended for a long distance on each side of the main spans. The wagon bridge, however, could have a relatively steep slope, so it landed on the ground right at the shoreline.
By the 1930s, automobile traffic has begun to overwhelm the capacity of the Interstate Bridge. There was talk of building a new bridge or even a tunnel under the harbor. The depression and WWII intervened. A new toll bridge was approved in 1953. The new High Bridge opened on December 8, 1961. Traffic preferred the free High Bridge over the toll on the Interstate Bridge, so traffic dwindled. The swing span was locked in the open position on December 21, 1962. Most of the now retired bridge was removed in the 1970s.
A portion of the Interstate bridge was thankfully saved. A small section of the trestle and one of the huge 325 foot truss spans still stands on the Duluth side of the harbor. It can be accessed by taking the Garfield Avenue exit off of Interstate I-535. The bridge has been modified to be a fishing platform. The ramp to the wagon deck is gone, and only a small section of the railroad level trestle remains. A pedestrian ramp has been installed through the center of the railroad trestle, a fishing deck has been installed under the truss span. This is the best that preservation fans could have hoped for given that the swing span posed a hazard to navigation traffic.
The dangers of a narrow swing span in a major harbor was illustrated on the early morning of August 11, 1906. At about 1AM, the steamship Troy signaled for the swing bridge to be opened. The captain of the Troy was unconcerned when the bridge did not start to move right away, the Interstate Bridge had a reputation of being slow to start to turn. As it was, the operator apparently was asleep at the post, and did not engage the bridge until it was too late. The Troy slammed into the swing span, pushing one side of the swing span into the harbor. The other side of the swing span collapsed moments later. Not only was the bridge destroyed and the harbor blocked, but 33 ships were trapped for the ten days it took to clean up the mess and open the passage. It took 2 years to rebuild and reinstall the swing span and return the bridge to service.
