I am from Portugal - which is a very peripheral region in Europe, bordering only Spain - but do speak several European languages, and one of my most interesting experiences in that sense you describe was in a train in Austria on my way to a ski resort, an intercity train (so, not even a long-distance "international" train) which was coming from a city in Germany on its way to a city in Switzerland just making its way up the Austrian-Alps valleys, and were I happened to sit across from two guys, one Austrian and one French, and we stroke up a conversation.

So it turns out the French guy was a surf promoter, who actually would often go to Ericeira in Portugal (were at a certain time in the year there are some of the largest tube waves in the World, so once it was "discovered" it became a bit of a Surf Meca) only he didnt spoke Portuguese, but he did spoke Spanish.

So what followed for a bit over an hour was a conversation floating from language to language, as we tended to go at it in French and Spanish but would switch to German to include the Austrian guy and if German wasn't enough (my German is only passable) we would switch to English since the Austrian guy also spoke it, and then at one point we found out we could both speak some Italian so we both switched to it for a bit, just because we could.

For me, who am from a very peripheral country in Europe, this was the single greatest "multicultural Europe" experience I ever had.

That said, I lived in other European countries than just my homeland and in my experience this kind of thing seems to be likely in places which are in the middle of Europe near a couple of borders and not at all in countries which only border one or two other countries.