I would recommend two spots:
1. Dorval's commuter terminal. You can park the car there, and watch CN, CP, and AMT Commuter trans. One tip: I wouldn't actually go on to the platform. If there's an AMT security officer there checking passes and he/she spots you, you will be ticketed.
2. From CN's St. Lambert station. If you don't mind the drive accross the Victoria Bridge, you
can get some pretty good railfaning in on Friday evenings at just around the supper hour.
Two months ago, the following were spotted betwwen 18;00 and 19:00 hrs:
-VIA's "Ocean" on its way to Halifax,
-AMTRAK'S "Adirondak" to Montreal
3. CN "double-stack" container train to Montreal from Halifax
4. CN mixed freight, heading to Montreal, 3 EMD SD untis and approximately 80 cars
5. AMT commuter train from Montreal to St. Hilaire
Actually, the order of above should have started with the AMT train. Once that comes through, standing on the platform is not a problem. Recently I was there to see a friend off to Halifax on VIA's latest version of the Ocean. The station manager did not mind at all, just so long as nobody got into mischief.
Hope this helps, sorry for the tardiness of a reply here, I only joined yesterday.
GPKTrainSpot