The bypass was intended to reduce congestion and separate passenger and freight traffic, and was designed for faster speeds and shorter travel times, saving ten minutes from Seattle to Portland compared with the previous route used by Cascades.

[5] 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.

[8] The 2006 Cascades corridor plan recommended that the curve and overpass where the derailment occurred be replaced with a straighter alignment, costing $412 million.

[10] At 07:33 a.m. local time (15:33 UTC), the leading locomotive and twelve cars of the southbound Amtrak Cascades number 501 passenger train derailed southwest of DuPont.

The train derailed while approaching the railroad bridge across southbound Interstate 5 near Mounts Road,[12] which contains a left-hand bend.

[17] The southbound train was operating from Seattle, Washington to Portland, Oregon, on the first revenue service run of the Cascades on the new, faster Point Defiance Bypass route between Lacey and Tacoma.

[18][19] Amtrak CEO Richard Anderson said that positive train control was not active on the track,[20] a factor cited in two accidents in Spuyten Duyvil and in Port Richmond, Philadelphia.

[31] The three passengers killed in the derailment were train enthusiasts, including two members of rail advocacy group All Aboard Washington.

[32][33] Amtrak temporarily suspended service for south of Seattle for several hours because of the accident,[26] resuming on the former coast route and the old Tacoma station.

[39] WSDOT announced on December 21 that it would not resume Amtrak service on the Point Defiance Bypass until positive train control was implemented in 2018.

[50] President Donald Trump said on Twitter a few hours after the accident that the derailment shows that his "soon to be submitted infrastructure plan must be passed quickly."

[51] The Associated Press and The New York Times reports of Trump's tweets said the accident had occurred on newly constructed track that was part of a recently upgraded line.

The lead locomotive recorder showed that the engineer had commented on the train's excessive speed six seconds before the derailment, and applied the brakes.

Because the engineer only had one operating trip in the dark traveling in the opposite direction, he had not established wayside landmarks to help him identify his location.

Such external cues are typically developed by operating crews after they become familiar with their train's territory, and without the use of landmarks, the engineer did not recognize that he had passed signs.

During this time he briefly looked outside, but misinterpreted a signal and returned his gaze to the control screens, finally recognizing that he had tripped the overspeed alarm.

The NTSB does not assign fault or blame for an accident or incident; however, it provided its determination of the probable cause of the derailment, deaths, and injuries,[1][2] the NTSB stating:The National Transportation Safety Board determines that the probable cause of the Amtrak 501 derailment was Central Puget Sound Regional Transit Authority's failure to provide an effective mitigation for the hazardous curve without positive train control in place, which allowed the Amtrak engineer to enter the 30-mph curve at too high of a speed due to his inadequate training on the territory and inadequate training on the newer equipment.

Contributing to the accident was the Washington State Department of Transportation's decision to start revenue service without being assured that safety certification and verification had been completed to the level determined in the preliminary hazard assessment.

Contributing to the severity of the accident was the Federal Railroad Administration's decision to permit railcars that did not meet regulatory strength requirements to be used in revenue passenger service, resulting in (1) the loss of survivable space and (2) the failed articulated railcar-to-railcar connections that enabled secondary collisions with the surrounding environment causing severe damage to railcar-body structures which then failed to provide occupant protection resulting in passenger ejections, injuries, and fatalities.