The Höllentalbahn (literally, "Hell Valley Railway") is a railway line that partially runs through the Höllental valley in the Black Forest of Germany. The line connects Freiburg im Breisgau with Donaueschingen, a distance of . Over its entire course the line rises from an altitude of in Freiburg to in Hinterzarten. Part of the route has a 5.5% gradient, making the line one of the steepest in Germany. The section Freiburg-Neustadt is electrified since 18 June 1936. Because of this and the steady decrease of traffic between Neustadt and Donaueschingen the route did not represent an operational unit anymore. The last direct connection between Freiburg and Donaueschingen ended in 2003 with the Kleber-Express Freiburg-Munich being discontinued. The missing section was electrified until the end of 2019 in the course of the project “Breisgau-S-Bahn 2020”, in order to enable direct trains from Breisach via Freiburg, Neustand and Donaueschingen to Villingen. History The line, which opened as far as Neustadt in 1887, was constructed for the Grand Duchy of Baden State Railways and was the last project of railway architect Robert Gerwig. The section of the line between Hirschsprung and Hinterzarten has a maximum gradient of 5.5%, and was initially operated as a rack railway on the Riggenbach system. The line was extended from Neustadt to Donaueschingen in 1901. In 1932/33, ten powerful Class 85 steam locomotives were built, which enabled traffic to operate over the Hirschsprung and Hinterzarten section without rack assistance, and rack operation ceased in 1933. In 1934, a new and partially tunnelled route within the city of Freiburg was opened. This enabled the original route, which passed closer to the city centre and over congested level crossings, to be closed. In 1936, the Deutsche Reichsbahn electrified the line between Freiburg and Neustadt, installing a 20 kV, 50 Hz AC. This led to the subsequent adoption of the 25 kV AC 50 Hz system in France and elsewhere. Since the electrification of Mannheim–Karlsruhe–Basle railway had been completed in 1958, the Höllentalbahn was linked to the nationwide electric railway system. Therefore, in 1960, the electrified section of the Höllentalbahn was converted to the standard 15 kV, 16 Hz system used throughout the German railway network. The Höllentalbahn was used successfully to test designs for the DB Class 144. Later, electric motive power was supplied by the Series 145, which was itself replaced by the DB Class 139. 1996–2019, the electric