A very complicated question.
I too would love for Amtrak to go to Bangor - I’m a regular user of Concord Coach from South Station to Bangor. It takes a little over 4 hours. I’m making the drive as I write this.
Tracks do indeed exist from Brunswick to Bangor but they are in horrendous shape. Upgrading those tracks to reasonable speeds so that the trip is at least somewhat competitive with driving would cost nearly a billion. Not to mention Bangor doesn’t even have a train station.
There have been various bills introduced to study the feasibility of this route. The proposal includes stops in Augusta, Waterville, and would use the old Maine Central ROW.
Like all transit related proposals in this country, it will take lots of time, money and political will, of which Maine has very little of those three.