MBTA's Providence/Stoughton line already provides quite a bit of service to the Providence airport, and improvements are underway to extend the line to Wickford Junction which should be open in 2012. Rhode Island is also funding an extension to this (which would probably be its own service called South County Commuter Rail) as far as Westerly which could connect to an extension of Connecticut's Shore line East commuter rail from New London which would provide continuous public transit from northern Massachusetts as far as Newark, Delaware, and if Maryland extends MARC up to Newark, you'll have non-Amtrak transit as far as Fredericksburg, VA.
Also, the existing MBTA equipment can already do 100mph I think with single diesels. This is also the segment of the Northeast Corridor that lets the Acela reach 150mph so excessively slower commuter trains would just slow it and the 125mph Regionals down more than they already do. Also, AEM-7s are no longer produced, you'd need with something based off of NJT's ALP-46a, or the new Amtrak locomotive, ACS-64 unless you want to buy used ALP-44s from NJT or used AEM-7s from Amtrak once they start retiring them.
A better extension of MARC is to Wilmington and shorten Newark-bound SEPTA trains to that station. MARC will stop at Churchman's Crossing between Newark and Wilmington.