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For almost an hour on Friday night, all TTC subway lines were shut down when the signal lights that guide train traffic stopped working.

TTC dispatched work crews across the city shortly after 8:40 p.m. to repair the signals that failed after “unknown communications breakdown.”

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The system that controls signals & allows us to see trains on all lines failed. This is very rare. We are working to understand the cause —

Brad Ross (@bradTTC) August 18, 2012

TTC spokeswoman Jessica Martin said it was a rare occurrence.

“We do apologize, but safety comes first.” Ms. Martin said if trains hadn’t been stopped, they risked a collision.

Supervisors rushed to local command points throughout the transit system so they could communicate with train operators via radio and guide them safely to the nearest station where passengers could exit.

Sincere apologies for this evening's subway disruption. At no time was safety ever compromised, which is why TTC institues a stop and stay —

Brad Ross (@bradTTC) August 18, 2012

Ms. Martin said she did not believe any passengers were stranded in tunnels for an extended period of time.

The signals, similar to traffic lights, were back up and running by around 9:20 p.m. Ms. Martin could not explain what caused the error, but said TTC officials are investigating.