The service suspension — which lasted from the time stations opened around 6 a.m. until just after 7:30 a.m. — was the result of a failure of all communication systems, the Toronto Transit Commission said.
The lack of subway service on four lines at a key time, combined with no replacement shuttle buses for the duration of the shutdown, left crowds of commuters stranded on city streets on a rainy morning.
The TTC explained that it didn't send out shuttle buses, which are normally deployed when a line goes out of service, because it simply didn't have enough vehicles to replace an entire subway system.
Even once subway service resumed, a backlog of transit riders at all stations led to delays and crowded trains throughout the morning rush.
A number of commuters took to Twitter to vent their frustrations, posting photographs of subway platforms teeming with people or streets clogged with commuters.
"TTC=Take The Car or Toronto Transportation Chaos," tweeted one person. "All TTC lines were closed and chaos has unleashed in Toronto," posted another.
Hours after the morning's hubbub, the head of the TTC made it a point to acknowledge the trouble the subway shutdown had caused.
"I absolutely understand customers' frustrations. I'm very disappointed that this happened," said TTC CEO Andy Byford. "Our reputation took a big hit this morning."
Byford explained that the TTC's main and back-up communications systems failed shortly before the start of subway service. The failure meant subway trains could not communicate with the TTC's transit control centre to go through tunnels safely.
"The safety critical system, which is the radio system to control communications with our trains, was lost," said Byford. "You cannot run a railway safely if you do not have the ability to communicate with the trains."
The transit authority then had trouble notifying customers about the subway shutdown because its email, Internet and phone systems failed as well.
"At the end of the day it's kind of ridiculous that the email and the ability to send alerts also went down just when customers needed it," Byford said. "Everything shouldn't be dependent on one primary system, so we are going to have a look at that."
The root of Monday's problems is being traced back to a faulty circuit board within one of the transit system's "uninterrupted power supply" units, which is used to filter power coming into the TTC's control centre.
"A failure within the UPS caused its battery system to drain, preventing power from getting to critical communications systems," the TTC said, adding that it was investigating the malfunction to prevent a recurrence.
Monday morning's subway shutdown also led to the city's Ryerson University and the University of Toronto to delay the start of their convocation ceremonies.
The incident, and its rippling effect across the city, led some to wonder if the shutdown was some sort of drill for an emergency preparedness exercise, particularly since Toronto is hosting the Pan Am/Para Pan Am games next month.
"I really hope the total #TTC subway closure was an emergency preparedness exercise," tweeted one person, while another posted "I bet this #TTC full system shutdown is an emergency drill getting ready for #PanAmGames."
The shutdown also came a day before a disaster management conference, which was to include a mock disaster exercise, was being held in the city.