At least the REM started out on a grander scale than the Skytrain. With SDO, a 4 car train could easily become 6. Then, as long as there is enough level station clearance, longer platforms should be possible.
Someday, SDO could even allow a 9 car Montreal Metro train to have 11 car trains like in NYC on the 7.
https://en.wikipedia.org/wiki/7_(New_York_City_Subway_service)#Fleet 11 car trains.
