Unfortunately, that's a bit of spin in the article. I have clipped the relevant part of the route network (http://imgur.com/a/OR4Ee) and the U55 (brown) connects Main Station (Hauptbahnhof) to Brandenburg Gate where it meets the main north - south light railway (dark green, pink). For reasons, this line doesn't have the best connectivity to Main Station, switching lines at Friedrichstraße is a chore, and U55 will be welcome for passengers of the north - south line.

In a couple of years, U55 will also connect to U2 and make it easier from passengers from some eastern districts to reach Main Station. It also will be the tourist line, connecting a lot of tourist sights in central Berlin.

There's a bit of background about the future gap closure here: http://www.projekt-u5.de/en/