The rails between Providence (soon to be Warwick as well) and Boston are already at nearly maximum capacity. Providence already sees 40-50 Amtrak trains daily http://www.amtrak.com/timetable/oct08/W04.pdf
. Add in all the MBTA trains and there is almost no room to add anything else, especially at peak times. Walk down to the train station at 5pm on a Friday, the tracks are heavily used. Unlike what Metro North has going down in New Haven-NYC with 3 tracks, there are only 2 tracks around here: a northbound and a southbound track. On top of it, Amtrak owns all the rails here and would have to approve any other service. I think MBTA was lucky to get the tracks down in Warwick as this is where there trains run at the highest speeds.