The S-Bahn Challenge in Berlin

Traveling an entire subway or S-Bahn network as fast as possible has become a kind of sport in many major cities, with increasing popularity. The Guinness Book of Records already has several entries in this category, all quite recent. Of course, the S-Bahn network of Berlin had been a blatant gap in this illustrious list. David, who is from Austria, immediately realized that determining the shortest tour is a combinatorial optimization problem and asked Martin Aigner of the Free University of Berlin, also born in Austria. He forwarded the request to the Zuse Institute Berlin, where it ended up at Isabel Beckenbach and Ralf Borndörfer. When Marc Uetz from the University of Twente suggested an Erasmus traineeship for Loes Knoben in Berlin, the topic was clear – we wanted to find the optimum round trip through the S-Bahn network in a joint Austrian-Dutch-German Arts-and-Math project.