I have had my doubts about how good these trains will be in the Montreal winters.

Guess that's one of the main reasons for testing, isn't it?

https://www.mtlblog.com/en-ca/news/montreal/the-first-rem-train-cars-are-officially-rolling-on-tracks-outside-montreal-video

The "representative segment" on the South Shore, as the REM calls it, "is made up of all the components of the automated system, which will be present on the REM's 67-kilometre network, as well as two stations, Brossard and Du Quartier stations, equipped with platforms screen doors."

The tests will make sure all those components, which include the communications, electrical and acceleration systems are running smoothly, the REM tells MTL Blog.

The train will first remain at low speeds to "test its functionalities and gradually test the interfaces between each of the components of the metro system." 

Testing will then ramp up and the trains will eventually reach speeds of 100 km/h.