The woman had boarded a Bay Area Rapid Transit train at Powell Station with her dog and as the doors were about to close she stepped out but the dog stayed on the train. As the train took off, she was dragged, pulled onto the tracks and killed Monday, BART officials said in a statement. The dog was not injured.
The woman appeared to be waving at someone moments before the train started moving, officials said.
"This is a tragic loss of life, and we are following all safety protocols," said BART Spokesperson Alicia Trost.
BART Police have reviewed surveillance footage and interviewed witnesses. BART's chief safety officer is working with the National Transportation Safety Board, which is assisting in the investigation.
Mike Sim, told KGO-TV, he saw the woman being dragged and spoke to a man who was hysterical on the platform and who told him the woman killed was his girlfriend.
The National Transportation Safety Board will assist in the investigation.
Our sister station KGO-TV contributed to this report.