Old Ludwig South-North Railway Tunnel Donauwörth

This old former railway tunnel, built in 1849, is the second oldest tunnel in Bavaria. Today you can only pass it on foot or by bike.

The tunnel is part of a very beautiful and attractive shortcut through Donauwörth for cyclists and has a very interesting history.
You can also cool down in the tunnel itself. But be careful: it suddenly gets dark when you enter because the tunnel is not lit. At least that was the case in the summer of 2019.
Update summer 2021: There was some light from a few fluorescent tubes, but it's still almost pitch black in there because you were driving through bright daylight beforehand and were pretty blinded by it 🤷‍♀️
The route of the old Ludwigs Süd-Nordbahn ran here from Nördlingen to the old train station and to the Danube.
If you want to know more, you can read and find out a lot here.

fahrplanspiele.wordpress.com/2012/08/09/auf-den-wegen-des-alten-bahn-hofs-in-donauworth-und-seiner-deckenfutterungen

Supposedly ammunition was produced here during WWII and Donauwörth was brutally bombed as a result, but apparently the actual production facility was not damaged, possibly because it was in the tunnel itself.
There is also a lot to be found on the internet about this.

This used to be the passage of the railway tunnel. Now pedestrians and cyclists can cross this long tunnel. You don't see that every day.