What is the best way to travel the US?

[BlackDiamond]

in 2023 will the MBTA extend to Springfield MA? Vermonter to Montreal? Can I travel in a circle from PortlandME to Springfield to Montreal to Halifax NS to lYarmouth NS to Bar Harbor ME to Bangor to Portland ME?

Highly unlikely that any of those will happen by 2023. That circle trip might work if you used buses as well as trains but it would not be easy and probably require overnight stays.