From Montreal east at that season, accumulating snow is unlikely but possible. Trees will be bare. In Fahrenheit, you could get up to 50 if the weather's mild, but 30s to 40s is more likely for highs, with lows in the teens to 20s. Blustery and cold is common.
On the Ocean, get up early for some views of the Chaleur Bay off to the left side of the train between Matapedia and Bathurst. I also love the marshlands in the vicinity of Sackville. It's a pretty ride throughout, if not quite as epic as the Canadian, though it's prettier in late summer when the wildflowers in New Brunswick and Nova Scotia add a lot of color to the landscape.