@leeloo@rauschma Not a good metaphor if you ask me. Those who see the ad (or graffiti) on the trains aren't the train owners, they're people who didn't ask for anything and see the train pass by (and train spotters of course, they're not there for the ad either), or passengers at the train station before getting on the train.
Ad networks pay site owners to show ads to whoever "passes by"; they come for the content, and see the ads (more like billboards, that don't move, rather than trains though I'd say).
I do use a blocker because of abusive behaviors. I wouldn't mind less intrusive and more secure and privacy friendly ads.