The lead locomotive and all twelve cars derailed while approaching a bridge over Interstate 5 (I-5). The trailing locomotive remained on the rails. A number of automobiles on southbound I-5 were crushed and three people on board the train died. The train derailed a short distance from where the new route merges with the previous route.
Preliminary data from the data recorder showed that the train was traveling at 78 miles per hour (126 km/h), nearly 50 miles per hour (80 km/h) over the speed limit, when the incident happened.
Overview of new Amtrak Cascades inland route (red; to the south), showing accident location, and old shoreline route (green; to the north)
The Point Defiance Bypass was built from 2010 to 2017 as a replacement for the BNSF mainline that runs along the Puget Sound coast between the Nisqually River and Tacoma. The $181 million bypass, using an inland route that follows I-5, was built by the Washington State Department of Transportation (WSDOT) on right of way owned by Sound Transit, the regional transit authority. The Amtrak Cascades service is a joint effort of WSDOT and Oregon Department of Transportation, with Amtrak as a contracting operator. In the wake of the December 18 derailment, the safety of the bypass was questioned by elected officials. The 2006 Cascades corridor plan recommended that the curve and overpass where the derailment occurred be replaced with a straighter alignment, costing $412 million. The final plans omitted the overpass replacement, with a smaller budget of $180 million granted for the entire project.