My two experiences with booze on a train:
In Chicago (lived there '98-'07), the western suburban commuter trains allow you to BYO alcohol as long as it's not in a glass bottle. As a result, the train station is filled with little bodegas selling beer cans and single serving plastic wine bottles. Also as a result, the experience at my office was that 75%+ of the western suburban commuters took the train. The dry northern and NWern burbs ran at roughly a 25% commuter rate.
In Denver, the ski train (lived here '07-present) to/from Winter Park serves beer and wine on the return trip. It is essentially impossible to book a ticket less than a month in advance -- and the trip is both expensive ($59 round trip) and long (2.25 hours, compared to 1.5 hours driving under normal conditions). It is a gorgeous ride, though.
Obviously booze on the train has potential problems, but especially when we're talking about the general post-work period during which few people will have had a chance to drink too much before they get in the train it isn't that big of a deal. A couple drunks on the train is preferable to a bunch of drunks on the road.