Subway service on the TTC's Line 1 between St. Clair West and Union stations resumed Thursday afternoon after being shut down for more than six hours, the transit agency says.

The problem was caused by the derailment of a work car on the southbound track at St. George Station at around 5:30 a.m., said TTC spokesperson Kadeem Griffiths.

Some 100 shuttle buses were running along that stretch of Line 1 during the period of the disruption.

Here's what it looked like at St. Clair West as frustrated passengers waited for shuttle buses:

Another derailment, this time of a work car, has forced Toronto's transit agency to shut down a large stretch of Line 1 on Thursday during rush hour 1:13

Commuter Elliott Turvis was on his way downtown for work. When he saw the crowds waiting for buses, he said, he thought "not this again."

"It's just a sinking feeling in my stomach," he told CBC Toronto.

Speaking to reporters at city hall, Toronto Mayor John Tory said there will be a "complete investigation into why it happened."

"I realized this caused a massive inconvenience for a very large number of people, and I'm sorry about that."

Riders wait in line for a spot aboard shuttle buses departing from St. Clair West Station. (Kate McGillivray/CBC)

To get downtown, riders were able to transfer to the GO Transit network at Downsview Park, Kipling, Dundas West and Union stations , as well the UP Express, without paying an additional fare.

This is the second time in less than a month that a derailment has caused a significant TTC subway disruption.