Toronto residents have been left wondering the cause of a train derailment that resulted in a four-hour chaotic subway shutdown last week, and now we finally know the answer. 

At a Toronto Transit Commission board meeting yesterday, officials revealed that the derailment was actually caused by "localized wear" of a section of rail in Keele Yard.

TTC officials said four trains had been dispatched from Keele Yard to go into service that morning, all of which were being sent out to the Bloor-Danforth line. 

TTC chief of infrastructure and engineering Fort Monaco says cause of derailment is believed to be "localized wear on a piece of stock rail."

— Ben Spurr (@BenSpurr) January 27, 2020

The fourth car of the fourth train then derailed slightly. 

"The preliminary conclusion is basically localized wear on a piece of stock rail," said Chief Infrastructure and Engineering Officer Fort Monaco. 

"The piece of stock rail is basically a fixed rail and then there’s a switch point that moves. The derailment occurred after the switch point and it was localized wear on the stock rail that we think, in essence, created the perfect storm."

He said it was a slight enough ramp to allow a relatively brand new wheel with a higher-than-normal coefficient of friction to basically climb itself up.

"Luckily," he added, "it was at low speed."

Monaco also said Keele Yard has been temporarily shut down while they work on short-term and long-term solutions. In the short term, he said the rail needs to be built up, shaved down and lubricated. 

In the long term, both the stock rail and switch rail need to be replaced entirely. 

Monaco said this should take about six weeks total, and that they're making sure "no stone is left unturned" in the rest of the yard so this doesn’t happen anywhere else. 

But while the derailment may have been slight, its impact on commuters certainly wasn't. 

Those travelling between Jane and Ossington stations on Line 2 were forced to wait for hours before attempting to board one of 116 crowded shuttle buses, with some residents even losing their jobs in the process.