To answer Mr. A320's questions above, in later years (1950-1960s), the B&M-CV-CN Montreal trains used the same route as Amtrak's Montrealer, following the CV mainline through Swanton to the border at East Alburgh and then running north on the CN via Cantic. The original Montrealer made no stops north of the border, but the day train (the Ambassador in the 1960s) stopped at Cantic, St. Jean, and St. Lambert on the CN portion of the route. The trains used CN's Central Station/Gare Centrale in Montreal and crossed the St. Lawrence on Pont Victoria, as did Amtrak.
Prior to the 1950s, trains used CV's now-abandoned St. Armand subdivision that left the mainline at Fonda Junction and ran up to the CN at St. Jean via St. Armand.