The Plochingen–Immendingen railway is a main-line railway line in the German state of Baden-Württemberg. It runs from Plochingen via Horb am Neckar to Immendingen, following the upper reaches of the Neckar from Plochingen to Neckartailfingen and from Kirchentellinsfurt to Rottweil, and finally the Danube from Tuttlingen to the end of the line. It was therefore also historically known as the Obere Neckarbahn (Upper Neckar Railway) or the Obere Neckartalbahn (Upper Neckar Valley Railway). The Rottweil-Immendingen section used to be called the Obere Donaubahn (Upper Danube Railway) or Obere Donautalbahn (Upper Danube Valley Railway). The Plochingen–Tübingen section is now sometimes known as the Neckar-Alb-Bahn (Neckar-Alb Railway), the Tübingen–Horb section as the Kulturbahn (Culture Railway), the Horb–Tuttlingen section as part of the Gäubahn (Gäu Railway) and the Tuttlingen–Immendingen section as the Donaubahn (Danube Railway). History Plochingen was connected to the network of the Royal Württemberg State Railways (Königlich Württembergische Staats-Eisenbahnen) in 1846 with the building of the Fils Valley Railway (Filstalbahn). The Upper Neckar Railway was extended from there to the south as follows: When it was being built, the line between Tuttlingen and Möhringen an der Donau, which was initially single-track, crossed the border from Württemberg to Baden and also passed through the Province of Hohenzollern near Dettingen. Württemberg therefore concluded a treaty with Prussia on 13 March 1865 and with the Grand Duchy of Baden on 18 February 1865, but built and operated the entire length of the line itself. At the border station of Immendingen at the time, there was a connection to the Black Forest Railway that had opened two years earlier. With the completion of the Stuttgart–Horb railway, the so-called Gäubahn, in 1879, the Plochingen–Horb section lost some of its importance because the direct connection from Stuttgart to the south ran through the Korngäu from then on. In contrast, the Horb–Immendingen section became increasingly important and, from 1900, had international express train services to Switzerland and, in later years, also to Italy. On 28 October 1899, the second track went into operation between Plochingen and Wendlingen (Neckar). From 1900 this extended to Neckartailfingen, from 1 October 1901 to Metzingen (Württ) and from October 1902 to Reutlingen. Later it was extended to Tübingen. The Plochingen–Tübingen section was electrified by 1 October 1934 for Stuttgart suburban traffic. On 23 and 24 October 1927, the state of Württemberg and Deutsche Reichsbahn signed a contract for the duplication of the Horb–Tuttlingen