AC, WS, & Porter have all had numerous "seat sales" this spring & summer. Its the 3 airlines which are the primary competition in the YUL-YYZ-YOW triangle.

With respect to your VIA Rail question, I find that VIA Rail often matches the base (Tango or equivalent) fare. (I don't think the reverse is true). Lately, VIA has been running more sales, with discounts as high as 75 %. As others have noted, VIA fares with taxes are much less than airline fares, due to landing fees, fuel surcharges, security fees, etc.

When I take VIA Rail, I particularly appreciate their WIFI - now complimentary in all classes of service.