After being gently rocked awake in her sleeper cabin, Sarah Marks spent the morning of her 29th birthday watching the Alps zip past the windows of her overnight train to Zurich. “The train comes in right next to the lake, with the mountains coming up behind it,” Marks said wistfully. “Very romantic, I have to say.” They join an increasing number of climate-conscious Europeans, particularly younger travelers, who are shunning carbon-spewing airplanes in favor of overnight trains.