One week after damage shut down a portion of the Orange line for half a day, the STM said it is clear that no object on the track caused the metro system to break down.

Laboratory tests conducted by engineering students at the Ecole Polytechnique show a "lateral force" on the track caused the damage to a train and an on-track signalling system.

What is causing that sideways force is not yet known.

Until it is, the new Azur trains will not return to service. The STM estimates it will be at least another week to determine the cause.

Over the past week the STM has sent the damaged pieces to an engineering school for analysis, equipped an Azur train with cameras pointed at the wheels and brakes, and run one train at full speed (72 km/h) from end to end of the metro system.

Seven of the eight Azur trains that were showing signs of damage have been completed repaired. The eighth train is the one that had the most damage, and the one involved in the Jan. 14 incident.

The STM said it will provide its next update on Friday Jan. 27.