The project is called the Point Defiance Bypass, and it was intended to shave some good time off the Amtrak Cascades route down to Portland. The Amtrak Cascades service is unique because the trains aren't conventional Amfleets and Horizon coaches. It uses special cars made by Talgo, and each car has it's own axle and wheels at one end. The car next to it provides the axle and wheels at the other end of the car. Whether the shared wheel arrangement played a role in the severity of the derailment is yet to be seen. There are no two-axle bogies, but the cars can be hooked up to any locomotives in Amtrak's fleet. Historically the trains had an F59PHI from a dedicated Cascades fleet pool at one end and an F40PH cabbage at the other, but then they got new trainsets for the service with cab cars built into the last car. Now it seems that they have locomotives on both ends.
It was announced later on that the Genesis was at the rear of the train, apparently undamaged, and that the Charger was in the front.
A frequent visitor to the Seattle-Tacoma area, I have driven under that bridge countless times. Interstate 5 is already heavily congested in that area most of the time due to Joint Base Lewis-McChord.