It happened around 12:30 p.m. on the Broad Street Line under City Hall.
Investigators say they now believe the woman climbed down onto the tracks and lay down as a train was coming into the station.
The train, however, stopped and the woman was not injured.
Police initially said the train passed over the woman, but it is now unclear if she was ever under the train during this incident.
The woman then got up and went over to medics after they arrived.