I just want to corroborate that you absolutely can purchase a ticket ex-BOS, BBY or RTE and board at any of those three stations. Yes they check tickets when you board at BOS, but only to confirm that you have a ticket for that train. If you have a ticket for that same train ex-BBY or RTE, they will not stop you from boarding. Also, if you purchase a ticket ex-BOS and do not board until RTE, your ticket will not be cancelled. I often do not know from which station I'll board when I purchase my ticket, so just will choose the station I think I'll depart from, but am never concerned, and never have had any problem. if I end up departing from a different station.

I also have never seen Amtrak police randomly check tickets on the platform, notwithstanding very extensive Amtrak ridership over the last 10 years (enough to be Select+ for most of those years). That said, I would not be the least bit concerned about being denied boarding.