There are several trainstations in berlin. Most trains will pass at least two of them, Bahnhof Zoo and Ostbahnhof. Ostbahnhof is the best station to go to, because it is only ca. 1500 meters (1 mile) away from the place where ph-neutral will be. Trains from the east (poland, russia, baltic states, ... will arrive Lichtenberg (only). See for more information about the trainstations below.
Ostbahnhof, is only ca. 1500 meters (1 mile) away from c-base (where ph-neutral will be).
From Ostbahnhof you can take a 3 minutes cab ride to c-base, or you take the S-Bahn to "Jannowitzbrücke" (leaves from track 8/9) and
walk to c-base (ca. 5-7 min.). It is also possible to walk to c-base in about 15-20 min.. S-Bahn will be 1.20 Euro,
cab will be around 6 Euros, walking will be free ;-). For more information check out the page telling you about your transport type.
If your train stops at Zoo, chances are high that it also stops at Ostbahnhof, so check it out and if it also stops at Ostbahnhof, stay seated and get out there.
If your train only stops at Zoo step out there, and take the S-Bahn to Janowitzbrücke.
If you arrive Lichtenberg step over to the S-Bahn there, go to Ostkreuz, step over there, and go to Janowitzbrücke.