The TTC suspended subway service from St. George to Lawrence West during the Friday morning rush.
Shuttle buses are running on Line 1, former known as the Yonge-University-Spadina line.
The service distuption, which started around 7:30 a.m., was due to a communication failure, the TTC said.
Ross said the power failure knocked out key parts of the system.
“Some safety-critical features — radio communication, chief among them [were affected]. If we can’t talk to the trains, and the trains can’t talk to the control centre, then we can’t operate service,” he said.
Ross stressed the closure was not caused by a signal problem and was not related to aging equipment.
He also apologized to riders on behalf of the TTC, and added that an investigation is underway to determine what triggered it.
“That’s why you do these investigations to find out what the root cause was, and then you work backwards and find out where did this originate and is there a problem somewhere in the system that we need to affect some repairs to, so it doesn’t happen again,” Ross said.
Seventy shuttle buses were brought in to handle the backlog.