An Amtrak passenger train derailed early Monday in Washington state near the city of Tacoma, with one car dropping down onto a busy interstate highway and some injuries reported, officials and witnesses said.

Amtrak confirmed the accident involving Train 501, which was heading south from Seattle to Portland, Oregon.

"Injuries and casualties reported," the Pierce County Sheriff's Department said in a tweet, but offered no details. Amtrak also offered no further information.

Photographs posted online from the scene showed a long passenger car dangling down onto the highway from an overpass, and other cars also off the rails.

The accident took place between Tacoma and Olympia, the capital of Washington state. Service on the line has been recently upgraded and was meant to be quicker, with improvements in tracks and signalling systems.