A bar car or snack bar wouldn't work on the 'T because 95% of the ridership is on too short a ride to make it worth bothering to get up for that drink. What might be a good idea, though, would be the sale of canned beverages in stations, explicitly for enjoyment on the train. They have that in L.A. and plenty of people enjoy a beer or three on their trains.
I have to admit, I would not particularly care to have people drinking around me on the train home. For one thing, those cars are crowded as hell -- no one wants a drink spilled on them. But also, not for nothing, between the crowds and the delays, frustrations sometimes run a little high. Yeah, most drinkers would keep their cool, but that one person loudly shouting at the conductor because the train is delayed... that's not gonna be fun for anyone.
(You should have seen the rider comments on the MBTA Rail app this morning when the Providence Line had a bit of a meltdown... eesh. And same thing whenever there's an evening delay.)
Maybe it could work on the after-peak evening trains? They tend not to be as crowded and generally have a more chill vibe to begin with. But, as F-Line says, there's non-trivial liability built in, which probably just tanks the whole concept overall.
Also, F-Line, that little backstory about the Montrealer versus the Adirondack -- great story, thanks for sharing!