The U4 is a subway line in Berlin, Germany. In 1903, the then-independent city of Schöneberg were in talks with the Hochbahngesellschaft (which operated the Berlin underground) to bring a line to their city. However, because the potential line was small, it promised little profit and negotiations stalled. Schöneberg decided to proceed with their underground railway alone and subsequently started construction in 1908. Schöneberg installed compatible tracks and trains which would allow possible future connections.
The U4 held the title of the shortest line until 2009 when the U55 was opened. The U4 line track has never been extended beyond its original length and no further extensions are planned.
I visited the U4 line outside of rush hour which meant a 10 minute wait between trains. These stations are pretty small and ten minutes was more than enough to cover them. Luckily, I didn’t visit during off hours, as the wait would have been extended to 15-20 minutes. I was glad to be done with the U4, it seemed to take forever.