Eurostar's alcohol policy is "Typically, we limit consumption on board to: 4 bottles / cans of beer or 1 bottle of wine per person. No large bottles of spirits."
It doesn't say whether this applies to alcohol purchased on the train, or your own alcohol that you have brought on board.
Once, when travelling from London to Paris, I was quizzed at security about the two mini bottles of red wine that I had puchased in the M&S at St Pancras.
In the United States, Amtrak's policy is "..Alcoholic beverages will be served on board trains in most Dining, Lounge, and Cafe Cars. You may also bring your own alcohol on board, though it may only be consumed in Sleeping Car accommodations..." They don't want people having late night parties on long distance overnight trains.
Back in the UK, I think that the policy of only being allowed to consume alcohol purchased on board is the best compromise. I do think though that if the journey time of the train is more than three hours, TOCs should make an effort to provide appropriate catering, maybe with alcohol only served at certain times of day, as happens in first class on Avanti West Coast.
People should also be allowed to carry alcohol that they have purchased before boarding on to the train, but in sealed containers that are kept well out of sight.