This is a favorite topic of mine. There are several multi-country metropolitan areas in Europe, for example Malmo-Copenhagen and Görlitz/Zgorzelec. Many cities have smaller suburb cities across the border, like Strasbourg-Kehl and Geneva-Annemasse. And the Benelux countries take this to the extreme, of course.

Even Berlin, which is pretty far from the Polish border, have commuter trains to Poland. That's right, the Berlin metro ticket will take you to Poland.