An equipment failure shut down Toronto's entire subway system during the morning rush hour, laying bare the city's reliance on transit and raising questions about priorities as council prepares to debate the future of the Gardiner Expressway.

All of the transit agency's communication systems failed in the wake of a power problem. The breakdown forced the TTC to stop its subways for 95 minutes and left staff – without phones or e-mail – struggling to convey the scale of the problem to their customers.

With more than 100,000 riders affected, it was a stark reminder of the key role the TTC plays in keeping the city functioning.

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Andy Byford, the CEO of the transit agency, said that the problems were not "specifically due to lack of funding," but warned that the system cannot keep going on indefinitely without major investment. The TTC has identified $2.7-billion in capital priorities that require funding.

"There's nothing on there that's [just] nice to have," he said. "These are all things that we need to be doing."

The subway shutdown came as another big-ticket item is about to come before council. What to do with the eastern portion of the Gardiner Expressway is set to be debated Wednesday. The two main options are replacing it with a boulevard for the long-term cost of $461-million or rebuilding the highway mostly the way it is for $921-million. The first option frees up more city land for development. The latter is better for some drivers.

Speaking Monday after a forceful speech extolling the need to keep the Gardiner as an elevated highway, the more expensive option, Mayor John Tory said that this does not preclude investing in transit, as well.

"I think we have to have both proper road transportation for cars and trucks and for commerce and for people just getting around, and much more public transit," he told reporters at the Empire Club. "We're going to invest in both."

But Jess Bell, the spokeswoman for the advocacy group TTCRiders, noted that the mayor has asked the transit agency to make cuts to its operating budget this year.

"When you look at how many people take the Gardiner East … you compare that to 125,000 to 150,000 people who catch the TTC and got delayed today," she said. "We think it would be far more sensible to look at cheaper, just as good, solutions for the Gardiner and reinvest that funding into public transit."

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According to the TTC, Monday's problems began with a pair of "uninterrupted power supply" (UPS) units, the primary and the backup, which filter electricity as it comes into their facility at Hillcrest. It appears that a faulty circuit board in a transfer switch caused a miscommunication between the two units.

"Power's coming through, which would normally recharge the UPS, but it drained more quickly than it was being built up by the regular power system," Mr. Byford explained. "And so progressively, the system just shut down. And that's when we lost our communications network."

The TTC chief said that a key priority was changing the system so that all forms of communication aren't vulnerable to a single failure. He explained that he was unable even to notify his board or the mayor by e-mail.

In a statement Monday, TTC chair Josh Colle called the shutdown "unacceptable" and said it was "deeply troubling that our system can experience such a major technical failure."

The city's two main subway lines carry about 1.25 million riders a day, according to TTC figures from 2013, and the effects of Monday's shutdown spread quickly. The volume of people shunted off the subway system was far too big to be served by shuttle buses, which weren't deployed.

Many people walked, with sporadic heavy rains making their mornings even less pleasant. Others crammed onto surface transit, but in some cases, the operators of these vehicles made the problem worse by not mentioning the shutdown at stops that intersected with the closed subway lines.

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Even after service resumed, stations were packed as the system had to absorb the backlog of travellers returning inside. According to TTC spokesman Brad Ross, it took at least another 30 minutes before the system was moving normally.

With reports from Tu Thanh Ha and Oliver Sachgau