A downtown relief line would have come in handy Wednesday morning when subway service in Toronto was suspended on a stretch of the Bloor-Danforth subway— also known as Line 2 — for three hours, the CEO of the Toronto Transit Commission says.

"It's not just a relief line, it's an alternative line," Andy Byford told reporters on Wednesday at the TTC Greenwood Yard.

Byford was speaking at an afternoon news conference announcing $150 million in provincial funding for design work on the downtown relief subway line. Mayor John Tory and Ontario Transportation Minister Steven Del Duca were also onhand to speak to reporters.

"Goodness knows, we could have done with that this morning when we had all those problems on Line 2, Byford said. Had we had that this morning, we would have diverted people onto the alternative line.

"Here, everything is binary. If you have Line 1 or Line 2 go down, pretty much you only have one option — shuttle buses."

Subway service resumed on Line 2 after a damaged feeder cable caused a three-hour service suspension between St. George and Pape stations early Wednesday morning.

ALL CLEAR: The delay at Yonge has now cleared and full service has resumed on Line 2 Bloor/Danforth with residual delays. <a href="https://twitter.com/hashtag/TTC?src=hash">#TTC</a> —@TTCnotices

The rush-hour commuting nightmare began around 5:15 a.m., when TTC crews working under Yonge station heard a loud bang, followed by smoke filling up the tunnel.

A small fire damaged a wood cover that runs over the third rail, and a feeder cable sustained what TTC spokesperson Brad Ross called "major damage."

More than 18 metres of the heavy-duty cable had to be repaired, said TTC CEO Andy Byford, and crews also had to check the switches and signals to ensure nothing else was damaged before service could resume.

When the fire first broke out, trains running on the north-south Line 1 had to bypass Yonge station, but quickly resumed normal service.

The stoppage led to a frustrating commute, with long lines of riders waiting for shuttle buses. At the peak of the stoppage, 90 shuttle buses were operating between St. George and Pape.

"I will always instruct that we suspend service and get the fire service in rather than put customer safety at risk," Byford said of the decision to stop trains.

"At the end of the day, inconvenient though it is, we are getting people from A to B."

The culprit. The burned wood is a cover board over the 3rd rail, which gives trains power. Cable being replaced. <a href="https://t.co/JswMxOMeUJ">pic.twitter.com/JswMxOMeUJ</a> —@bradTTC

West-end passengers who needed to get downtown were able to travel by GO or UP Express with TTC fare. West-end travellers could start their journey at the Kipling GO station, while east-end travellers were able to use the Danforth or Kennedy GO stations.

Ross had warned of longer than typical wait times on the west end of line 2 because the stoppage had trains "trapped" in the Greenwood yard, which is east of Yonge.

Smoke fills the air outside the Yonge-Bloor subway station Wednesday morning. (Tony Smyth/CBC)

Now that the cable is repaired and service has resumed, the TTC will investigate what caused the cable to degrade and begin smoking, Byford told reporters at Yonge station.

"Something will have set that off," he said.

Meanwhile, paramedics were called to treat two TTC workers for smoke inhalation. They were treated at the scene, and did not need to be transported to hospital.