Thanks for the information which is exactly to the point.

However, from a Berliner's perspective this is currently the most useless subway we have. It is not linked at all to the subway network and will not be for the next few years (maybe for ever). To get from Berlin Hauptbahnhof to Brandenburg Gate I do not need to undertake the hazzle of a subway trip but simply jump on a TXL bus which does the same trick overground an might take two minutes more of travel time (but which is easier to navigate and does not force me underground). If you want to go further than Brandenburg Gate you need to do this anyway.

As you said, they plan to connect this U55 to the U5 from Alexanderplatz to Hörnow (a route most tourists would never use). However, we still cannot be certain when or even whether this will happen at all.