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“Localized wear” of a section of rail at the Toronto Transit Commission‘s Keele yard is believed to be responsible for a train partially derailing on Wednesday, causing a major hours-long shut down of part of Line 2.

The preliminary findings of an investigation were shared during a TTC board meeting on Monday.

Officials said as the subway service day was beginning Wednesday morning, four trains were being sent out to the Bloor-Danforth line. The fourth train set to enter service at Keele station encountered a piece of rail with “localized wear” after a switch point.

READ MORE: TTC subway service resumes after part of Line 2 was shut down due to partial train derailment

Chief Infrastructure and Engineering Officer Fort Monaco said “a perfect storm” then followed.

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“A slight enough ramp to allow a relatively brand new wheel … (to) basically climb itself up,” he said.

“Luckily it was at low speed.”

READ MORE: TTC CEO discusses efforts behind the scenes after partial subway derailment

It took TTC staff several hours to get part of the fourth subway car in the train set back on the rails and to restore service to Line 2.

In total, 116 shuttle buses were brought in from various routes to provide service between Jane and Ossington stations. Toronto police closed part of Bloor Street West as thousands of morning rush hour commuters affected by the closure flooded the streets.

Monaco said TTC staff are inspecting the entire yard for similar issues, noting the affected section was shut down.

As an interim measure, he said crews need to build up the damaged piece of rail and shave it down. Within five to six weeks, Monaco said the rail section and switch will be replaced.