Toronto's subway system was briefly shut down because of signal problems on Thursday morning.
The Toronto Transit Commission announced the disruption around 9:30 a.m. ET. Service was restored within about 15 minutes, but was slower than usual.
Delays were expected for about an hour while the TTC cleared a backlog of trains.
"Rarely do we ever have to shut down the entire system," Mr. Ross said. "But when we're unable to move trains or see the trains, signals and switches that is a very significant safety concern."
Mr. Ross said the transit agency is still trying to pinpoint what caused the computers at transit control to fail.
He said a similar computer malfunction prompted a five-minute shut-down on the Yonge-University-Spadina line earlier this week.
Thursday's glitch left controllers temporarily blind to the locations of trains and unable to manipulated switches remotely, prompting to officials to halt every train on the Yonge-University-Spadina, Bloor-Danforth and Sheppard lines.
Toronto's subway cars have drivers, but they would literally be in the dark if they sped through tunnels without transit control to guide them.
The shutdown did not affect the Scarborough rapid transit line.