I was in Berlin earlier this week and rode the U55 on the first (free) day of service, when there were huge crowds of joy-riders. On Monday, the first day of normal weekday service, the trains were well patronized, with what appeared to be many tourists (with luggage) and office workers (with briefcases). I also can't see much purpose in the line right now, but as one poster noted, the city of Berlin would have to repay federal construction funds if it hadn't started in service. The same applies to the "Stammbahn," where federal funds were used to build a connection to the north-south mainline leading to the new Hauptbahnhof. The impoverished city/state now doesn't want to rebuild the "Stammbahn" (a line through south Berlin from Potsdam to the main station), and the federal government is asking for its money back.