Originally Posted by
WWads
LGA-BOS works because Amtrak has to share with Metro North, and the Acela is frequently delayed during rush hour as a result.
If Amtrak made its slots on time, they wouldn't get put in behind MNRR locals. On the other sections of the line, Amtrak regularly makes commuter trains wait for delayed Acelas. (When I commuted on NJ Transit for about 18 months, we were held for a late running Acela to pass probably once a week on average.)
But even putting that aside, the best case for NYP-BOS is about 3h30 due to the numerous curves, low speeds, and outdated infrastructure through much of the NEC from roughly New Rochelle to New London. That's just slow enough to be less competitive with flying for many trips.