Police say the woman, who is believed to be in her 30s, was walking across the tracks at 15th Street SE and East Pioneer Street and was hit by a Sounder train carrying 250 passengers.
Witnesses told police the woman did not attempt to avoid the train.
The train was briefly held at the scene, but has since been allowed to continue. The passengers were picked up by buses. Vehicle traffic is being rerouted around the crash site.
Police are investigating the incident as a suicide.