Late Tuesday, Germany's national rail network stopped. Every Deutsche Bahn train across the country stopped abruptly wherever it was, going nowhere. The culprit - a failure in GSM-R, the Global System for Mobile Communications for Railways. It's the backbone of how train drivers talk to traffic control centers. When it goes down, trains don't move. Not because there's a safety threat in the conventional sense, but because no communication means no authorization to move, and rail safety protocols are unambiguous about that. You sit and wait.