Hm... as of 10 minutes ago, Washington-Boston roundtrip train fares are $120 (cheapest, getting a 14-day discount on the return, not on Acela). But flying from Baltimore into Providence is only $108 (again, with a 14-day discount on the return flight) on Southwest, or about $149 with everyone else. Flying from IAD or DCA is prohibitively expensive (>=$350 RT). If you decide on air or rail, we will happily split the rental car costs...