Heiligenberg Tunnel
The 422nd longest tunnel in the world
The Heiligenberg Tunnel is the longest of a total of twelve tunnels on the Mannheim-Saarbrücken railway and the longest in the Palatinate. The tunnel crosses the Palatine Watershed in the German state of Rhineland-Palatinate. It was originally built for a single track, but a second track was built a few years later.
Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).