From 27C3 public wiki
Yankee's ride
I bought train tickets from Cologne to Berlin with the following trains:
Köln -> Alexanderplatz 26.12. 18:48 - 23:28 Alexanderplatz -> Köln 30.12. 18:34 - 23:12
(A&O Hostel has 24h desk)
I paid 69.40 Euros for the tickets to Berlin and back. I have five tickets to spare. If you want to have a ticket please contact me. I sell the ticket for the price I paid during Novemer. After that I will ask for 75 Euros for taking the risk of buying the tickets.
If I run out of tickets I may be able to reorder some, but the price the Deutsche Bahn charges might increase.