First of all, this thread with thanks to the various news media has gone from commuter to Amtrak so I've moved it for now to Miscellaneous.
We know now that this will in fact be usage of Shuttles equipment. How they are using them is different on weekdays and weekends. On weekdays, 2 Shuttle trips 495 & 471 will originate in Greenfield in the morning, and 2 Shuttle trips 494 & 478 will terminate in Greenfield late at night. What I don't know yet is whether they are being stored overnight in Greenfield or deadheading to and from Springfield. Given that the trip by the new timetable takes 1 hour 8 minutes, it seems silly to deadhead equipment and crews back to Springfield only to go back to Greenfield in a few hours. To ride these trips one could stay somewhere in Greenfield overnight.
On weekends, they are definitely storing the trains in Springfield, because there are short trips just from Springfield to Greenfield 400 (which then turns as 461 which now originates in Greenfield) and Greenfield to Springfield 499 (which is the return of Shuttle 488 equipment after it terminates in Greenfield). The weekend trips allow an up and back journey with a very short layover in Greenfield, but one must either leave Springfield at 7:50 AM, or arrive back in Springfield at 9:18 PM. If you don't live in the area it will require staying in a hotel in Springfield.
This is being touted as a pilot program, however they are forgetting an important market. All those colleges and universities in the Northampton and Holyoke areas could use a northbound train in the morning and a southbound train in the afternoon or early evening. So there may not be enough service being offered to gauge success.
What's outlined above can now be reserved on Amtrak's site. For now the only actual timetable is the one linked to. It will be interesting to see how Amtrak handles this, since they have never put out a timetable solely for the Shuttles; it's always been buried in the New England-Washington version of the Northeast timetables. Perhaps the Hartford Line schedules will show this too.