The TTC says service has resumed on Line 2 after a portion of it was shut down when a subway train car leaving a yard partially derailed Wednesday morning, causing commuter chaos in the west end of the city.

Line 2, between Jane and Ossington stations, was closed in both directions just after 6 a.m., and didn’t reopen until more than four hours later.

“This was a bad morning for our customers travelling into and out of the west end and for that we apologize,” TTC spokesperson Stuart Green tweeted.

“Thankfully, subway derailments, even partial ones, are rare, but we commit to finding the root cause to avoid a repeat.”

About 100 shuttle buses were running between the stations along Bloor Street. Thousands of frustrated passengers bundled in winter coats were forced to wait for the buses, with many ended up being late for work because of the huge delay.

Police closed Bloor Street West between Royal York Road and Jane Street, and only allowed TTC shuttle buses to pass through because the road was congested with pedestrian traffic. The road reopened at about 10:20 a.m.

“Our crews are reporting a partial derailment of one of the middle cars of a train that was leaving Keele Yard. It was not in service,” Green tweeted earlier.

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Green later tweeted that the TTC hoped to have service running by lunch hour before amending his prediction.

“Service resumption within 30 mins,” Green tweeted at about 9:50 a.m. “Root cause analysis underway. Important to learn from these incidents.”

The TTC then tweeted at about 10:17 a.m. that service had resumed.

It’s the first major unintended shutdown of service on the TTC this year, but just the latest in the last few months.

On Dec. 2, almost 200 people were evacuated from a train at Dundas West station after a wooden board touched the third rail, sparking a fire.

On Nov. 7, a cable came into contact with a fence on the Prince Edward Viaduct, causing an electrical short.

On Oct. 22, private contractors accidentally drilled through the concrete subway tunnel liner, causing a three-hour shutdown of parts of Line 1.

Miriam Lafontaine is a breaking news reporter, working out of the Star’s radio room in Toronto. Follow her on Twitter: @mirilafontaine