The commotion started on the platform and ended on the tracks at the 53rd Street E train station in Manhattan.
NYPD Detective Leo Pereyra was off-duty on Wednesday. He heard screaming and saw people running to the edge of the platform.
Some surveillance video of the area was made available and you can see from the PIX11 video that riders knew something was wrong.
Detective Pereyra, a 21-year-veteran of the NYPD, ran toward the crowd and saw the woman jump on to the tracks.
He was able to get her back on the platform and asked MTA staff to contact 911.
If this situation occurs, the MTA says riders should immediately call 911 and notify station crews at Help Point kiosks located on platforms and seek transit personnel or police.
The installation of WiFi at all stations has brought cell service and more of the intercoms with the blue lights to the system's 472 stations.