Recently (well, two years ago already...) a new "main" railway station was opened in Berlin. It is quite impressive, both in its size and architecture.
It is located in a very center of Berlin - you can easily walk from it to the Bundestag and Branderburger Gate. Not many big cities can afford (space-wise) to build something so big completely de novo in their city centers. However, as the photo below demonstrates, in the center of Berlin space is not an issue: