The woman had a dog leashed to her waist when she boarded a train at the Powell Street station on Monday. As the doors were about to close, she fully got off the train, but the dog did not.
The woman appeared to be waving her arms to get someone's attention as the train took off. She was then dragged and pulled onto the tracks.
The dog was not injured. There was no visual indication that the dog was a service animal, but that is under investigation, BART officials said.
"This is a tragic loss of life, and we are following all safety protocols," said BART Spokesperson Alicia Trost.
The National Transportation Safety Board will assist in the investigation.