Way back in the 20th century, Boston's passenger rail service coalesced into two mostly independent systems; a rail network north of the city, servicing the northern suburbs, and the same in the south. The southern system was the northern terminus of the intercity passenger railroad system to New York and beyond. These systems are seperated by about a mile of dense urbanization.

There was discussion during the planning of the "Big Dig" infrastructure project of the 1990s to join these systems via a tunnel, but this was rejected due to the technical complexity, and more importantly, the cost, with the politicial battle around the project as a whole.

We could have had, for example, through service between Lowell and Providence, or between Newburyport and Worcester. Amtrak could have continued the Northeast Corridor into Maine.