Police and medics responded to the scene, at 5th Avenue and South Holgate Street, at about 7:15 a.m. after receiving a report of a pedestrian hit by a train.
The victim, a man believed to be in his 50s, suffered heavy trauma and was rushed to Harborview Medical Center in critical condition.
Investigators are trying to determine the sequence of events that led up to the accident.
A drug recognition expert responded and evaluated the train operator, which is standard procedure in an investigation such as this. The operator showed no sign of impairment, and was released to Sound Transit supervisors, police said.
The light rail tracks and South Holgate Street were closed between 4th and 6th avenues during the investigation, but have now reopened.