TTC Subway service resumed on the Yonge line between Union and Bloor on Monday morning after a lengthy shutdown.
The TTC tweeted trains had started running again around 8:20 a.m., not long after the agency warned the stretch of subway line could be closed until noon.
Some 60 shuttle buses ran up and down Yonge Street while the trains were out of service, though TTC officials recommended people people use the University subway line to stay warm on an extremely cold morning.
TTC Spokesman Brad Ross said the problems started when a Toronto Hydro substation flooded around 3:30 a.m., cutting power between King and Union.
Toronto Hydro, however, tweeted that the problem was due to TTC-owned equipment failing.