The incident saw 13 cars of the Amtrak Cascades 501 become derailed near DuPont and tumble from the bridge to the highway Interstate 5 during rush hour traffic.
The accident involved five vehicles and two trucks, reported Reuters.
Several motorists on the road were also injured.
The news agency quoted Washington State Patrol spokeswoman Brooke Bova as saying that approximately 100 people were subsequently taken to nearby hospitals, including ten people that have sustained serious injuries as a result of the incident.
Amtrak stated that there were around 80 passengers and five crew members on-board during the accident.
Cranes have now been brought to clear the debris from the site, while the US National Transportation Safety Board (NTSB) has been called to investigate the incident.
The derailment was noted to have happened on the first day of operation on the new inland route between the Washington cities of Tacoma and Olympia.
In addition, the Amtrak Cascades Trains 504 and 509 that were scheduled to run on the same route were cancelled following the incident, though services from Seattle to points north and east continues to operate.