Speaking as a person who drinks, and thinks a lot of rules and laws concerning alcohol are over the top, this is kind of disturbing. I would never even consider brining open alcohol on the monorail, for the simple fact I would not want to spill it on anyone. I don't even bring open sodas on there. Why would you need too? They don't let you bring it out of the gate at EPCOT, they dont sell it at MK. So basically, you would be brining it either in cans from your room or in plastic cups from the monorail resorts. Is it that hard to finish a drink before you get on?
Or am I miss-interpreting what you meant. Did they used to prevent closed alcohol like a bottle of wine or liquor or a pack of beer, or do you really mean they will let you bring open alcohol on the monorail?