The TTC has identified a tiny bump in a section of rail as the culprit behind a train derailment that caused hours of delays and commuter chaos during last Wednesday's morning rush.

The offending piece of rail was found at the TTC's Keele Yard, where trains are dispatched to Line 2 Bloor-Danforth.

"It was localized wear on the stock rail that we think, in essence, created the perfect storm," said Chief Infrastructure and Engineering Officer Fort Monaco at the TTC's board meeting on Monday.

The wear on the rail created a small ramp, Monaco explained, which resulted in a train wheel coming off the tracks after it climbed the incline on Wednesday morning.

The partial derailment shut down service at six stations on Line 2, which resulted in massive delays and long lineups for the shuttle buses dispatched to serve the route.

The TTC has closed Keele Yard while it makes repairs to the section of rail.

Crews will either shave it down or lubricate the section of rail in the short term.

The TTC plans to fully replace the section of rail within the next six weeks.

Monaco said investigators will also "leave no stone unturned" as they examine the rest of Keele Yard for rail issues that could cause further problems for trains.