A train engineer saw the body at about 5:15 a.m. on the tracks near 5th Avenue South and S. Spokane Street and called 911.
Detectives arrived and found a woman dead on the tracks. They said it does not appear she was struck by the train, and police consider the death "suspicious," said Seattle police spokesman Mark Jamieson.
Police are not yet saying how they believe the woman was killed.
The tracks have been closed as the investigation proceeds.
This is a developing news story. More information will be posted as it becomes available.