Given that this is reasonable transit pitches, and not crazy transit pitches, I will reiterate what
@TheRatmeister said. That’s an unreasonable transit pitch based on cost-benefit.
Acela trains will reach top speeds of 165 mph (265 km/h) when new trains enter service, and 186 mph (300 km/h) in coming years. Other services reaching 125 mph (200 km/h) are prevalent in the US, and are officially classified as
higher-speed rail. The ceiling of what I would remotely consider reasonable at this point for Portland - Boston would be something loosely in the “higher speed rail” category. A good parallel is the Harrisburg - Philadelphia corridor. Service like that could terminate in Portland in the north and point(s) south of Boston on its southern end.