The STM has discovered exactly what was causing damage to the Azur trains and how to fix it.

The Azur trains were pulled from service two weeks ago when one train was badly damaged near the Du College station.

They will start running again on Saturday Jan. 28.

That incident shut down metro service on a portion of the Orange line for much of one day, and prompted the STM to inspect all trains for signs of damage.

It found that every Azur train was showing signs of damage to a runner, or electrical contact boot, that rides on the third rail.

Following two weeks of investigations and test runs with cameras attached to the bottom of trains, the STM learned that wear on the tracks was causing damage to the trains.

The director of engineering at the STM, Francois Chamberland, said 50 years of wear has created grooves in some rails, and while it's not a problem for the older trains, the newer Azur trains have different contact points.

"The groove itself is not the problem. It's the side of the groove that needs to be smooth so that when we get a more used shoe... it will not make a hard contact and keep it from moving to the side," said Chamberland.

As a result at certain portions of the track the runners on Azur train were being pushed to one side -- and being subject to a "lateral force" that was causing damage.

The STM is now taking steps to fix the damage, and to limit wear and tear in the future.

Welders are going to work on the tracks and make sure they are within the tolerances needed for the Azur trains.

The runners on both the older and newer trains, which are normally serviced about every 12 months, will be inspected often, and likely be replaced after nine months.

The Azur trains will also be fitted with an attachment to reduce damage in the case of another severe failure.