Commuters were left in the lurch when service was disrupted for about an hour at the peak of rush hour.
The orange line ground to a halt between the Berri-UQAM and Montmorency metro stations around 7:30 a.m, although the rest of the network was working as usual.
The STM confirmed paramedics had to be called to the Laurier station.
The transit agency scrambled to provide extra buses for those stranded, and Laval's transit service ordered its buses to make detours to take commuters onto the island.
This is the second major disruption of the metro system in less than a week.
Computer problems paralyzed the entire network last Tuesday in the middle of the afternoon rush hour.