Makes sense to begin as a Newport Flyer summer service. The 2002 study projected 576 daily visitor trips through the corridor on weekends, which would probably push a little higher if you removed the cross-platform transfer penalty and ran a train straight from Boston.
The last thing anyone who actually lives in or visits Newport regularly wants is more day trippers waddling up and down the sidewalks debating between the t-shirt or the refrigerator magnet to commemorate their visit.