Cool thought experiment, but not viable or feasible or ... imaginable.

Realistically this single train might take 24 hours to complete this route with all of the stops and turns and speed issues going through densely settled urban and suburban towns. The elevation changes would be dramatic for any train line. That Gardner -> Worcester leg goes just about straight over Wachusett Mountain and the whole valley there, it'd be a very dramatic change in elevation, or a very, very expensive tunnelling through a mountain, for an area that is already pretty well served by state highways. And I just can't imagine anybody would want to go from Gloucester to Plymouth by way of Worcester, pretty close to a hundred miles out of the way.