In Berlin and in Frankfurt you can rent a bike all over the city per "Call a Bike".
You register by mobile stating your bank account or your credit card number. If you find one of the bikes parked near the main junctions, you phone in using the number you find on the bike and receive the code to open the lock. A service hotline registers parked bikes and tells you where they are. You find detailed information in English at www.callabike.de
Partners of the German Railways rent bikes in many places, e.g. in Muehlhausen , Erfurt , Weimar , Dessau and Heiligenstadt .
You obtain information (mostly in German)
by clicking into www.bahn.de (click into "Int. Guests" for English information),
on the bicycle hotline of the German Railways 0 18 05-15 15 15 (Euro 0,12/minute),
in the brochure "Bahn & Bike"
(download here from www.bahn.de , available at train stations) or
from the German Cycling Association "Allgemeiner Deutscher Fahrrad Club" www.adfc.de .