Twitter user Benjamin TD, a software engineer and mapmaker, created an application that shows users what areas of Western and Central Europe are accessible from within 5 hours of a given train station. Countries that have spent a lot on high speed rail, such as France and Germany, have enormous coverage. For example, as you can see, almost all of Germany is accessible within 5 hours of Frankfurt.
The heat maps show how close different areas are, with darker colors representing closer stations. Even London is accessible within 5 hours of Calais, thanks to, I suspect, the Chunnel.
The Øresund Line crosses the Baltic Sea gap between Denmark and Sweden, making traveling between those two nations easy.