Switzerland opens the world's longest and deepest rail tunnel through the heart of the Alps in an engineering marvel that stands as a symbol of European unity at a time of increasing fragmentation.

The 57.1-km-long Gotthard Base Tunnel, 17 years under construction and designed to last a century, is part of a 23 billion Swiss franc infrastructure project to speed passengers and cargo by rail under the mountain chain that divides Europe’s north and south. The project that federal transport office director Peter…