The train stalled between the LaSalle and Charlevoix stations on the metro's green line shortly after 8 a.m.
It was evacuated, and commuters had to walk alongside the train through the dark tunnel to reach the next station.
The incident caused a backlog at several other stations, but a spokeswoman for Montreal's transit authority says the service was reestablished quickly.
STM spokeswoman Isabelle Tremblay said the outage took place on one of the system's older trains.
Tremblay said she did know how many commuters were involved in the incident, but she said there are are an average of 1,000 people on green line trains during morning rush hour.