Berlin Hbf. (main station) is now connected also by subway line U5 which connects via Brandenburg Gate and 3 new stations under street "Unter den Linden" to Alexanderplatz and beyond, e. g. Frankfurter Tor or Tierpark.
More info about the project of building the new part of U5.
Full report with map by local public TV (in German). Tip: Use DeepL for translation.