The video, posted on Facebook last week by "remadxrealcitylive", has more than 25,000 views. It shows a woman lying on the Metro tracks, with blood on her head. Three men jump down and pull her up onto the platform.
The STM told CBC Montreal that the incident happened at 11:47 p.m. on March 12.
A commuter used the emergency phone at Jarry station to alert authories that a woman fell onto the tracks.
A train operator also noticed something on the tracks, alerted authorities and cut the power.
The incident lasted only a few minutes.
The woman was taken to hospital.
It is unknown how she ended up on the tracks. The STM says it is investigating the incident.