The M Ocean View-line train hit the woman at about 9:45 p.m. near the intersection of San Jose Avenue and Farallones Street.
Muni spokesman Judson True said the woman reportedly was under the front left corner of the train, but that the wheels did not make contact with her.
Police had initially reported the injuries appeared to be life-threatening, but authorities have since said the victim is expected to survive. She was taken to a local hospital for treatment.
Buses are taking customers between Broad Street and Plymouth Avenue and San Jose and Geneva avenues.
San Francisco police and the San Francisco Municipal Transportation Agency are investigating the incident.