Donautalbahn
When the Royal Württemberg State Railways put into service the first section of the Danube Valley Railway between Ulm and Blaubeuren in 1868, construction by the Baden State Railways on the Schwarzwaldbahn in Baden from Singen to Offenburg had reached an advanced stage. On 15 June 1868, two days after the opening of the Danube Valley Railway section, the Black Forest Railway section between Engen and Donaueschingen was also opened for service, which, between Immendingen and Donaueschingen, follows the course of the Danube. Württemberg, which was following the progress of the Black Forest Railway construction with great interest, now formulated the goal to connect their rail network to the new line in Baden. However, building an extension to the Danube Valley Railway from Sigmaringen to the connection point in Immendingen in Baden, which would have meant the early completion of the line, was not in the plans. Instead, Württemberg wanted to extend the Upper Neckar Railway, which parted ways from the Fils Valley Railway near Plochingen, and then reached Reutlingen, Tübingen, and, in 1867, Rottenburg am Neckar, with the extension going through Horb and Rottweil, and further through the Neckar valley in a south-westerly direction to the border between the two states, to establish a connection to the Black Forest Railway there. In addition to a route between Rottweil and Villingen, another railway was built from Rottweil to Tuttlingen, which was completed on 15 July 1869. Württemberg then further constructed the connecting line from Tuttlingen to Immendingen through the Danube valley, which was completed on 26 July 1870. This meant that the section between Tuttlingen and Immendingen was completed as a connecting line to the Black Forest Railway, and the section between Immendingen and Donaueschingen was completed as part of the Black Forest Railway.