Building light rail over a moving, floating interstate has turned a tad trickier than engineers originally envisioned.  

As the Seattle Times first reported, Sound Transit potentially needs an additional $20 million for Parsons Brinckerhoff to design rail tracks over the Interstate 90 floating bridge, a key connection between Seattle and points east. The change increases the firm’s contract from $36 million to potentially $56 million, still a relatively small expenditure in the larger scheme of corridor changes.
 
In order to install light rail over a floating bridge, engineers must determine how custom joints in the track can handle the fluctuation of weather and Lake Washington conditions, everything from waves and wind to water levels.
 
The voter-approved $3.7 billion East Link line has a target date of opening in 2023, but ever since the ballot measure was approved in 2008 engineers have put a focus on figuring out just how to do everything from attach the steel rails to the concrete bridge—epoxy is the likely answer—to how trains can handle the movement of wind and waves, especially with multiple trains potentially on the bridge at the same time.

The University of Washington will enter the fray, beginning a study of how train vibrations may impact the bridge.

The Times reports that Parsons Brinckerhoff has made solid progress on the engineering so far, though, with testing of track in Colorado showing that trains can run safely at 55 miles per hour on the bridge, for example. Sound Transit and Parsons Brinckerhoff have also established track changes needed to improve comfort when trains transition from ground to bridge.

Sound Transit still expects construction on the existing bridge to start in 2017 and adding light rail to it isn’t one of the first steps in the construction timeline.

Tim Newcomb is Engineering News-Record’s Pacific Northwest contributor. He writes for Popular Mechanics, Sports Illustrated and more. You can follow him on Twitter at @tdnewcomb or visit his website here.