Sonny Jackson, a spokesman for Denver police, said the woman appeared to be in her mid 20s, The Denver Post reported. Her identity had not been released late Tuesday.
An eyewitness told the Post the woman appeared to be drunk. Ryan Collins, a construction worker, said the train stopped for the woman but then the operator moved off without opening the door.
"We saw the train move, and she started tumbling around," Collins said. "Her legs were under it, she got dragged under it."
Jackson said the woman had been standing in the middle of the tracks, waving at the train until it came to a halt at a place that was not a designated stop. By the time the train began moving again she had walked around to the side.
The train stopped again about 20 feet from where the woman had been struck.