The TTC says the combination of an old rail and a new wheel led to last week's train derailment that caused a delay for thousands of commuters, that was unfortunately overlooked by inspectors.
Engineer Fort Monaco explained the cause during a transit presentation Monday night, where the wheel hit the rail, which had "localized wear" after a switch point.
"That we think in essence created the perfect storm," he said. "A relatively brand new wheel with a generally higher than normal coefficient of friction, basically climb itself up."
He said thankfully the train wasn't going that fast and the rail will be temporarily repaired until it's fully replaced over the next five to six weeks.
The derailment happened around 6 a.m. forcing a closure from Jane to Ossington station for the entire morning rush, sending thousands of commuters to the street to get on shuttle buses.
As for how this was missed, TTC spokesperson Stuart Green explained the train came out of its Keele yard, which isn't as busy as its other ones.
"Our focus is on the larger yards and on the main line and we just may have missed that during an inspection, but that's no excuse, that's just the reality," he said.
The yard has been shut down until the temporary replacement is done, with increased checks on other yards to make sure there's no risks for a similar event.
"We haven't had a main line derailment since 2008 and so that's the good news, but we need to do better," he said, adding that goes not only for the initial derailment, but the coordination of services on the street, as well as communication.