The MBTA and RIPTA seem to be considering having cross-functional fares with Amtrak. So you could use your MBTA pass to ride the Amtrak which is great because it means that Amtrak would act as an express train between Boston and Providence, while the MBTA can focus on frequent local service. I guess a downside is that there might be days where a lot of people will have to stand on Amtrak trains unless they somehow figure out a system with the seating.