When I discovered that seats on Switzerland’s panoramic Bernina Express trains were sold out for my May travel dates, I panicked.   Then I began researching other luxury trains like the GoldenPass Express, which travels between the snow-glazed peaks of Interlaken and the lakeside beaches of Montreux.   I also studied the schedules of regional and intercity trains that crisscross the country, connecting cosmopolitan cities with mountain villages.   You’ve probably heard about “travel dupes,” or less crowded alternatives to European cities like Barcelona and...