Why the train takes an hour and a half to cover about 15 miles at most and gets there far too late to have a chance to tailgate/go to Patriot Place/watch warm-ups is one of the mysteries of the MBTA that only a proper Bostonian could answer. But it does get you there from central Boston and the atmosphere on it, I've found, is cheerful and friendly. I don't know who said that bags can be taken in to the stadium but are searched -- that wasn't my experience two years ago (had to leave a backpack in a tent by the entrance). Rugs and so on carried by hand are fine. Your plans for dealing with the cold make sense to me -- the stadium is built so that it's like being on an open hillside in the rain or snow.