Maybe a suicide, or accident, but because of a person on the tracks, half the S-Bahn trains (s5, s7, s75 and s9) around Hackischer Markt did not go this morning. So if you need to go to work maybe find a replacement U-Bahn, or a bus.

More at Tagesspiegel:
http://www.tagesspiegel.de/berlin/nachrichten/verkehr/83614.asp