I don't think all Americans realize what it's like over there. (Or for that matter, over here in some parts of the US.)
I went to Europe in 1989-90, (supposedly) to attend the University of Bonn. Public nudity wasn't prohibited at all in many European countries. There were naked people on regular broadcast TV, much more naked than the "partial nudity" which was on American broadcast TV for a few years in the mid-90's, starting with "NYPD Blue." Public topless sunbathing occasioned barely a batted eye -- except of course on the part of people visiting from prudish places like much of the US. The changing areas at public swimming pools were segregated by gender, men and boys over here and women and girls over there -- but they were not enclosed. There were great big T&A posters in the main train station downtown, bigger than life-size.
By the end of my year in Bonn, with some excursions to Paris and other places, I had seen so much public nudity, photographed and on video and irl, without having had to go the slightest bit out of my way to see it, that it had become kind of boring. Also, the viewer doesn't get to pick which people go around naked. So you see some stuff that nobody wants to see.
There's an annual day in London where big crowds of people get naked, frolic in fountains etc. I think it might be summer solstice. One gentleman was (is? I don't know if he's still around) somewhat famous for going about his business every day in London carrying a briefcase and umbrella and wearing nothing but a bowler hat, black socks and black shoes.
Somewhere in rural Germany in the 1990's there was a bodybuilder who liked to go around completely naked. Some local prude complained and complained until the police felt forced to arrest him. This case made nationwide headlines -- not because the guy was going around naked, but because the bodybuilder, who was generally thought of as a harmless eccentric, had been arrested for no good reason. I don't understand the legalities involved here, whether public nudity actually is illegal in Germany, but the law is hardly ever enforced, like the laws in some American towns requiring horses to wear pants, or what.
That's how things were in the 1990's, at least. I suppose it's possible that since then, Europe has become more prudish. But not bloody likely, I would guess.
I should not be talking about the entire continent at once like this, because some European countries actually are more prudish than others. I would imagine that in some parts of Europe, you can get into trouble for taking off your clothes. It's just that in other parts -- you can't.
So if you're naked on a train in Europe, be careful if the train crosses a international boundary into a more prudish place.
And of course: some American cities and regions are much more "European" than others. Apparently there is no prohibition of public nudity in California, for example, unless a local lawkmaking body imposes such a prohibition. Now and then, here and there in the US, groups of people go naked in order to protest something, or in the interest of art or what have you. In some places they are arrested, and in other places, even if local laws prohibit public nudity, those laws have not been enforced.