October 1927, the Free People's State of Württemberg and the Reichsbahn signed a contract, which set out the terms of the expansion of the Gäubahn. Württemberg provided the Reichsbahn with a credit line of 35 million Reichsmark, and the Reichsbahn agreed to expand the section between Stuttgart and Tuttlingen to be twin-tracked, and also to construct a single track connecting line to the station in Hattingen on the Black Forest Railway (Baden). This construction phase began in 1928. On 15 May 1934 the Reichsbahn opened the 8.2 kilometer-long single-track connection between Tuttlingen and Hattingen. Until construction stopped due to World War II in 1941, the Reichsbahn was also able to complete the expansion of the line to twin tracks from Stuttgart to Tuttlingen, with the exception of a 2.3-kilometer-long piece near Oberndorf am Neckar. With the completion of the single-track connection to Hattingen and the now twin-track Black Forest Railway (Baden), an efficient railway between Stuttgart and Singen had been created. The trip via Immendingen, and the requisite train change at that station, were therefore eliminated. As part of the expansion of the line, the Reichsbahn also expanded the node stations in Rottweil and Horb am Neckar, and replaced the old station buildings in Tuttlingen and Eutingen with new construction. The expansions during the 1920s and 1930s resulted in great improvements in the service schedule on the Gäubahn. Express trains traveled from Berlin via Erfurt, Würzburg, Stuttgart, Zurich, and Milan all the way to Rome. The last peacetime schedule in 1939 showed 3 express trains from Berlin to Italy, even though not all of the trains continued from Milan to Rome. In addition there was express service between Berlin and Lucerne as well as between Stuttgart and Konstanz. Travel time between Stuttgart and Singen was cut to 2 hours and 41 minutes in 1933, and the regional and local service schedules also saw marked improvements. As was true prior to the railway expansion, and since World War I, the Württemberg class C steam locomotives were used in express service. After the completion of the expansion, it was the Prussian P 10 which assumed the lead on these trains. The Württemberg class C was reassigned to manage the regional and local service traffic. The class Württemberg T 5 was also still seen, but was replaced by 1933 by the DRG Class 24. The Prussian P 8 had already been used since