Nice follow-up. During my short research, I was surprised to see they make a specific carve-out for drinks from an open container! I guess it makes sense, as the train might stop on short notice. Also, if you really think about it, it is weird to travel on a train drinking something from an open container(!). It would normally have a lid/cover.

I’ve generally understood open containers to include drink containers with lids, at least for legal restrictions.

https://en.m.wikipedia.org/wiki/Open-container_law

> The possession of cans, bottles or flasks or other vessels containing an alcoholic beverage could potentially result in a violation of open container laws.

> To be “open”, in most cases, means that some of the contents have been removed, the seal is broken, the cap is off or the alcohol is otherwise readily accessible.

Even the wording above makes it sound as if interpretations can vary, though.