The TTC has identified the suspected cause of the subway derailment that temporarily shut down service and wreaked havoc on thousands of residents’ morning commutes last Wednesday: a half-inch protrusion on a piece of rail.

At a meeting of the TTC board Monday, the agency’s chief infrastructure and engineering officer Fort Monaco said the investigation into the incident is ongoing. But the TTC’s preliminary conclusion is that the culprit was “localized wear” on a piece of track at the agency’s Keele Yard.

The derailment, which occurred around 6 a.m., forced the TTC to shut down service on Line 2 between Jane and Ossington stations for more than four hours in the midst of morning rush hour.

The agency deployed 116 shuttle buses to pick up stranded riders but they struggled to cope with demand. In parts of the city’s west end, crowds of commuters grew so large they spilled into the streets as they waited to board a bus. Police had to temporarily close Bloor Street between Royal York and Jane streets.

In an interview, Monaco explained that over time a short section of track at Keele Yard had become misshapen, causing a bulge on the inside of the rail. He said the size of the protrusion was between three-eighths and one-half inch, and extended for about nine or 10 inches along the rail.

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According to Monaco, the bulge effectively created a small ramp that the wheel of one subway car travelled up as it was being moved out of Keele Yard to enter service, causing the subway to slip off the tracks.

“Luckily it was at low speed,” Monaco said, pledging the TTC would “leave no stone unturned in the rest of the yards to make sure this isn’t happening anywhere else.”

Three subways had already been moved out of Keele Yard without incident that morning, and the problem only affected one wheel on the fourth train.

Monaco said he could only speculate on why only one wheel was affected, but this particular wheel was new, which in conjunction with the wonky rail may have created a “perfect storm” that caused the derailment.

That’s because older wheels are more likely to be worn down and sit slightly less flush against the rail, making them less prone to catching on such a small bulge.

Monaco said the rail at Keele had likely become so worn down because a piece of equipment called a kick plate that’s supposed to guide trains’ wheels over the tracks had been missing from the yard since 2015.

He said he didn’t know why it went missing, but it has to be custom made and isn’t readily replaced. The problem may have gone undetected because Keele, which holds only four trains, is much smaller than the TTC’s busy Greenwood and Wilson Yards, and gets less attention from staff, Monaco said.

The TTC has shut Keele Yard while it deals with the problem rail. The agency plans to shave off the protrusion and reopen the yard by the end of this week, and replace the misshapen track entirely within six weeks.

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Subway derailments are rare for the TTC. Monaco said prior to Wednesday, there hadn’t been one since 2008.

He said agency staff regularly check the condition of “mainline” tracks used for passenger service, and riders can be confident the system is safe.

“People should not be worried,” he said.