The train fell over Interstate 5 near Mounts Road between Lakewood and Olympia.
The southbound lanes of Interstate 5 were completely blocked by the train and motorists were being warned to avoid the area.
Seattle Times quoted Pierce County Sheriff's spokesman Ed Troyer as saying that there were fatalities on the train and that motorists had been injured.
According to media reports, this was the first time the train was running down a new bypass created to avoid slow curves and single track tunnels on the BNSF Railway main line tracks near Point Defiance and along southern Puget Sound.
Further details are awaited. (ANI)