Ridgefielder wrote: Was Boston - Montreal service ever offered via Portland in the past, via either the Grand Trunk or the Mountain Division?
To the best of my knowledge, no, and I'm reasonably cognizant of New England railroad history. There was definitely no through service between the Grand Trunk and Maine Central... GT trains terminated at their own station next to the GT office building that's still standing today. The MEC Mountain Division was strictly a local operation in modern times (which I'm defining as the mid-20th century onward). MEC trains from Portland did connect with the CP at St. Johnsbury, and the B&M-CP Boston-Montreal service via White River Junction and St. Johnsbury lasted into the mid-1960s, a decade longer than MEC passenger trains. So Portland-Montreal via St. Johnsbury with a change of trains was a viable route for many years. I'm reasonably sure there was never any B&M-MEC through service between Boston and StJ via Portland, and definitely no Boston-Montreal through service via that route. B&M-MEC through trains all headed northeast at Portland, not northwest.
But it's always possible that there was some through train that ran for two weeks in 1910 or something that I never heard about.