I've always been confused about the 100mm bottles. 5 guys in cahoots equals 500mm and that's OK. 1 nutter with 125mm is a massive threat. I think not. I understand it is now OK to take a steel knife on board with blade the length of a credit card; so the security guys in UK told me. My Swiss Army is such a knife with scalpel sharp steel. It's OK, but my yoghurt? oh no.
I once watched as the water bottles and other liquids were passed through the X-ray machine to be then used to extract the hard earned dosh from the pax on the other side. I asked how the x-ray detected the water. Silence. I then asked that if it was OK for the WH Smith water to be x-rayed and approved why not mine. Even more silence. How come the Airline passenger association is not asking the same question.
The perceived threat will always be more effective than the act. You mention the trains & tubes; what about the Chunnel? It's almost as bad as 'reds under the beds' and look where that led in UK & USA.