The derailment happened at about 8:50 a.m. as the train was heading to Seattle, the Seattle Times reported.
Mud, rocks and other debris slid down a cliff and struck the last three cars of the train, said BNSF Railway spokesman Gus Melonas.
No one was injured during the incident and passengers from the derailed cars were transferred to other cars to continue on to Seattle.
Efforts to clear the tracks of debris are under way, Melonas said.
As a precaution, BNSF issued a 48-hour moratorium on passenger trains on the railway's main line; freight trains will still be able to access the line.
Marc Magliari, an Amtrak spokesman in Chicago, said he could not say how many passengers were on the train at the time of the incident, which he described as "a relatively low speed event."