I just went there today
a lot of trains on that place,we can easyly pass from AMT platform to VIA platform,but we have to get by an underpass to get off of it on the bus terminal,just across,and from there,you can get on the VIA platform,just by walking across the bus path to the platform,as you can see on the map..
but a most better place,if you want to see CN and CP more closer,it's at Lachine station,you have to get it at the end of the 48th avenue in Lachine,it's around 2 miles from Dorval,may be less,easyly reachable by Victoria street on a direct way to get to the 48th and get to Lachine station,by walking at the west end of the platform,you got a nice view from both sides in both ways,I used to go there on the summer,or at Beaurepaire overpass,where is a great place for railfans,also at the Xing at Beaurepaire AMT station a bit farter on west,by stabding between the two line,you get a good view too
Now I just show one shot or two from Dorval today
at Dorval station on the week end,there's always some railfan who can share you some tips and some spots to see
I also recommand to try the Montreal/Delson AMT line,on the week.on noon time,you can get a departure with a possible come back,the only one..
from thi strain,you can be lucky and be able to acces in the cab of the control car and get some spectacular shots like this one