My favourite are picturesque Harz Narrow Gauge Railways.
Longest single network of narrow gauge railway in Germany, with a total length of 140.4 km (87 miles), 44 stations and halts. Its trains run daily to a timetable and it operates more than ten steam locomotives, seven diesel railbuses and three trams.
The most famous part is Brocken Railway wich
runs the highest mountain of the Harz at 1,141 m (3,743 ft)
https://www.youtube.com/watch?v=C6QUB4LrPew