Taken from Getting Around Germany website.
It’s worth nothing that Germany is a union country, and transport strikes (Streik) can occur at any time, although it is still a fairly rare occurrence and there is usually plenty of advance notice. Still, travelers should beware that even a short “waing strike” (Waungstreik) in one city can ripple through the entire system, causing long delays and even cancellations. On these days, be prepared to adjust your travel plans and wait longer than usual. An interesting note about these situations: since trains and punctuality are so important in the German world, the DB will hand-out official “certificates of train tardiness” (Bescheinigung über Zugverspätung) if a train is late. You can use these as “excuse slips” for arriving late to work, school, or other appointments.