Speed, human error blamed for Amtrak derailment near Chambers Bay.An Amtrak spokeswoman said the train’s engineer was removed from his post following the accident.
Update from the Seattle Times.
About 100 yards before the Chambers Bay bridge, the northbound passenger train passed a final signal that, when red, tells the engineer to stop because the drawbridge is being raised or lowered. The train then moved through a derail device that knocked it off the rails before reaching the bridge, Amtrak said.
Very similar the the Jersey Central Newark Bay Drawbridge accident in 1958:
History.com - Train Plunges off Bridge
New York Daily News Flashback - New Jersey train plunges off a bridge into Newark Bay killing more than 40 in 1958
Wikipedia - Newark Bay rail accident