According to the STM service updates website, service on the Green line will be down until 9 AM.
A power failure is the apparent cause, but there are few other details.
TLDR: The entire Green line is down because of an apparent power failure. Service is so far projected to resume at 9 AM. No details on the cause of the power failure.
Shutdown GREEN line between Berri-UQAM and H-Beaugrand. Power failure. Service expected to resume at 9:00. #stminfo G
The STM tweeted the above notice to passengers at 8:29 AM, meaning they anticipate needing half an hour to right the electrical error that caused the power failure.
Given the length of the shutdown, it likely more than just a tripped breaker.
Oui, un service provisoire métro est en cours sur ce tronçon entre Berri-UQAM et Angrignon.
If we come across more details we'll update you when we can.
UPDATE: The STM has sent out this tweet for passengers:
[Temporary bus service] Temporary bus service is being deployed between Berri-UQAM and Honoré-Beaugrand. You can find the bus stop localization on the neighbourhood map displayed in all Metro Stations. https://t.co/51WrxCYjSl
A shuttle will be running between Berri and Honoré-Beaugrand while the Green line is down.
UPDATE: Service is gradually resuming on the Green line, according to the STM.
The service is gradually resuming. The interval between the trains will therefore gradually return to normal.