After waiting a moment for the mechanisms to do their work, you step out of the elevator onto the first subterranean level of the Hauptbahnhof. On this level is the "1". People in the city will just say "the 1" when they're talking about this particular underground train line. Technically speaking, this is only one of six or seven underground train lines that connect the city. On the UWU manuals distributed to the Underground Workers' Union, this particular line is notated as the "U1", but after the north-south regional was developed, the "U" designation exited popular usage, and the "U1" became the "1", as it is commonly known now. You'd really need a map if you were going to get on the 1 though, especially since you don't really know where you're going yet...