Banning alcohol =/= banning drunks
By banning alcohol on all public transport, you wouldn't be stopping drunks abusing or threatening people. That said, there is a part of me that agrees with the idea of banning alcohol on public transport (mostly for the safety and comfort of other passengers); with regard to trains though, a line would have to be drawn - TfL are probably right in banning it on their services as a journey on the Tube is akin to a bus journey in terms of duration and style, but there have been times when I've had a beer on a long-distance train.
However, by banning people who have had alcohol from public transport, to be honest, that would be a very bad idea - there have been times when I, and I expect many other forum members, have had drinks of an evening, and have had to rely on public transport to get home again, be that a bus or a taxi. There is a difference though between a 'comedic drunk' (which I generally am) and an 'abusive drunk' - the latter is the one that you would need to curtail from public transport, but I understand that a good number of public transport providers will have, as do shops, pubs, and nightclubs etc., a 'right to refuse service' to people who are drunk or who it is feared will cause a danger to other people. (Admittedly, a refusal of service could cause more problems with abusive drunks!)
Basically, my point is that alcohol is not abusive drunkenness, so a blanket ban would be negative for the myriad of people who may want to sit down on a train and enjoy a drink, and wouldn't necessarily stop the people for whom alcohol causes them to be abusive and violent.
Do I know what the answer is? No. But a blanket ban isn't, I don't think, it.