evidence: This summer’s air travel chaos has added another reason why a growing number of European travelers are opting for the rails. But a key factor—and the one behind a number of public and private initiatives to bring back overnight trains—is concern over climate change. Journeys by rail can emit as little as one fifth the greenhouse gasses as flights, which is why railway companies, national governments and E.U. bodies cite reducing emissions as a reason for re-establishing a network of overnight medium-distance rail lines that once connected the continent. Yet for all the demand and effort, those initiatives still have a long way to go.