This airport has it's own railway station, which is within walking distance of the airport terminals
(400 m or 5 minutes).
The Airport Express
It seems that in 2010 the train needs:
- 17 minutes to reach Berlin Ostbahnhof
- 21 minutes to reach Berlin Alexanderplatz,
- 28 minutes to reach the main railway station.
These seems to be special dedicated trains, which seem to be available every half hour.
During daytime there seem to be also plenty of other trains available, with even some service during the night.