The project seems exciting, but a Montreal based blog suggests that the REM will not have sufficient capacity to handle existing traffic especially in the morning peak hours.
The problem is that the REM will use existing tracks and will displace existing commuter services such as the Deux Montagnes line where the exiting trains are much bigger and longer.
For details see http://www.cat-bus.com/2016/05/how-the-caisses-light-rail-system-will-crumble-under-its-own-weight/