funny, I did use a flask for a bottle years back on an Amtrak trip to Winter Park. Buddies used some empty 7-up bottle(s) for Vodka.And the thermoflask? Did you decant first, or drink it straight from the bottle?
Asked Amtrak ahead of trip if carry on liquor/beer was ok, they said "technically no unless you have a cabin, but we don't search unless people give us reason to"
We were all very discrete after hitting the bar car once it closed. We then walked to back of train to explore and caught site of a big Harley type dude with a Bud 30 pk under his arm distributing it to his friends.
I hadn't worried much since then about carry on.