The rules say I can drink any type of alcohol anywhere on the train so long as I buy it from Amtrak.
I figured that out quickly. The other heavy rail system I take regularly is Caltrain, and they still have a policy where alcohol is allowed except for "special event" nights after 9 PM. This would normally be the day of San Jose Sharks or San Francisco Giants games. They don't sell anything on board, so obviously it would have to be one's own supply. I've seen groups with cases of beer. I've brought my own - typically 24 oz or less. I talked to a conductor about it, and he says that the stainless steel in the vestibules is relatively easy to clean. That's where some drinkers go after they've had 3 or 4 too many.
As for the beer on Amtrak - I really don't know about paying $5 for a 12 oz bottle of beer when I could get a six pack for just a little bit more and bring it in myself. Do the LD trains have beer on tap? I've only had breakfast and didn't ask about alcohol. Would they even serve alcohol with breakfast? A nice Sunday Champagne brunch sounds nice.