FYI from 3 June all flights to Berlin will go the new BER airport in the South of the city. All other airports will be closed. Trains to Berlin Hauptbahnhof will run every 10 to 15 minutes and journey time will be 30 minutes. BerlinLinienBus.de e.g. will serve places like Dresden directly from the new BER airport.
The new Willy Brandt airport will also have a railway station. I'm just not sure it will open together with the airport or not.
The rail system map for the VBB (Vehrkehrsverbund Berlin-Brandenburg) shows a station at the airport with multiple regional and S-Bahn lines. The VBB shows S-Bahn connections from Berlin Hbf to the airport starting June 3rd. The Bahn website shows regional trains from Berlin Südkreuz to BER starting the night before.
Thanks for the latest information. Good to know since I'll be back there this summer. One can take a bus from BER to Dresden....great!