There's an article here, albeit from 2015, listing places and companies, including train companies, that allow it and have banned it - https://www.eliquidukstore.com/2015/09/places-e-cigarettes-banned-uk-vaping/
Lets say on a packed West Upper concourse before a game, 200 people are vaping away, but one person, somewhere, possibly toward the direction of the Harvey's kiosk by W3G - it's difficult to know for sure- is smoking a real one. Who is this miscreant and how can he be easily identified for...