The Baden main line (German: Badische Hauptbahn) is a German railway line that was built between 1840 and 1863. It runs through Baden, from Mannheim via Heidelberg, Karlsruhe, Offenburg, Freiburg, Basel, Waldshut, Schaffhausen and Singen to Konstanz. The Baden Mainline is 412.7 kilometres long, making it the longest route in the Deutsche Bahn network and also the oldest in southwest Germany. The section between Mannheim and Basel is the most important northern approach to the Swiss Alpine passes, whilst the section between Basel and Konstanz is only of regional significance. The stretch from Karlsruhe to Basel is also known as the Rhine Valley Railway (Rheintalbahn) and the Basel–Konstanz section as the High Rhine Railway (Hochrheinstrecke).

The Upper Rhine Valley has been an important trade route from Central Europe to Switzerland and Italy since Roman times. With the development of railways in the early 1830s, considerations arose of building a railway from Mannheim to Basel in order to handle the movement of people and goods faster and cheaper than was possible with carts, especially as the Upper Rhine south of Mannheim only allowed restricted navigation. George Stephenson’s locomotive and the opening of the line between Liverpool and Manchester was first discussed in the Baden Parliament by the priest and liberal politician Gottlieb Bernhard Fecht (1771–1851) in the autumn of 1831. He was accused of being ahead of his time and his proposal received no support. The first real initiative for a railway was made in 1833 by the Mannheim businessman Louis Newhouse, but just like the suggestion of Friedrich List, it was not supported by the Baden government.

It was only when a railway company was founded in neighbouring Alsace in 1837 with the goal of building a west bank line between Basel and Strasbourg that Baden began to plan the building of a railway to avoid a shift of the traffic flow on the Rhine to the Alsace shore. A specially convened meeting of the Baden Assembly of the Estates (Badische Ständeversammlung) decided in 1838 to build a railway from Mannheim to the Swiss border near Basel at public expense, as had been called for on 31 July 1835 by the Freiburg historian Karl von Rotteck. The line would serve as the main line of Baden and would therefore have a generally straight route on the eastern edge of the Upper Rhine Valley. Baden was the only German state to choose initially to build its new rail network with 1600 mm broad gauge.

Construction began as soon as September 1838 on the first section between the two new terminal stations in Mannheim and Heidelberg. After two years of construction, this section was officially opened on 12 September 1840. The continuation of the line to the south took place in the following stages: Heidelberg–Bruchsal–Karlsruhe on 10 April 1843, Karlsruhe–Ettlingen–Rastatt on 1 May 1844, Rastatt–Oos (now Baden-Baden station) on 6 May 1844, Oos–Offenburg on 1 June 1844 and Offenburg–Freiburg on 1 August 1845. All the station buildings on the line from Mannheim to Freiburg, some of which have been preserved, were planned by the Baden architect Friedrich Eisenlohr. It was followed by the Freiburg–Müllheim section on 1 June 1847, Müllheim–Schliengen on 15 June 1847 and Schliengen–Efringen-Kirchen on 8 November 1848. The ongoing construction to the Swiss border was disrupted by the events of the March Revolution and the course of the line was damaged at several points by guerrillas. The completion of the section to Haltingen was delayed to 22 January 1851. Once it became clear that all the neighbouring countries had chosen standard gauge for their railways, the Grand Duchy of Baden State Railway (Großherzogliche Badische Staatsbahn) rebuilt its lines and rolling stock to standard gauge within a year in 1854/55.

While the law of 1838 only referred to the construction of a route "from Mannheim to the Swiss border at Basel," discussions turned to the question of an appropriate end point and the connection to the Swiss rail network. While the city of Basel called for a connection to Basel, there were other suggestions in Baden. There was scepticism about a foreign end point for the line, so the Baden town of Lörrach or even Waldshut were preferred. The political debate was dominated by many national arguments and reservations. So, in 1846, Baden granted the Swiss Northern Railway Company (Schweizerische Nordbahn, SNB) the concession to build a railway from Basel/Lörrach to Waldshut, continuing through Switzerland to Zürich in order to achieve a connection with the Swiss railway network in Waldshut in Baden rather than to Basel in Switzerland. However, the Swiss Northern Railway Company could not muster the financial resources to build the line, so the license lapsed. Only the Baden-Swiss treaty signed on 27 July 1852 could achieve a durable solution for the continuation of the Baden main line: the treaty provided for the further construction from Haltingen on the east bank to Basel and continuing through Bad Säckingen to Waldshut. This led to the unusual situation, which still persists, of a German state railway company building and operating a railway line in another country.

The route from Haltingen to Basel was opened on 20 February 1855, followed by the section to Bad Säckingen on 4 February 1856, continuing on 30 October 1856 to Waldshut, where, with the opening of the bridge over the Rhine on 18 August 1859, the first railway connection was made between Baden and Switzerland.

While the focus during the construction of the Mannheim–Basel/Waldshut railway was on the international north–south traffic and the connection of the largest cities of Baden, the Baden-Swiss treaty already provided for its continuation with a rail connection towards the Lake Constance area and Konstanz. But disagreements on the route led to the delays on the approach to Schaffhausen: while the Swiss side preferred a route through the northern Klettgau on Swiss territory, Baden favoured a route through Jestetten in Baden. It could not, however, insist on its preference and the continuation of the main line from Baden to Konstanz, opened on 13 June 1863, ran from Waldshut not through Jestetten, but via Erzingen and Beringen. This last section, directed by Robert Gerwig, completed the construction of the Baden main line.

Already in the Act of 1838, the construction was planned for two branch lines: the Appenweier–Kehl line, connecting to Alsace, was opened on 1 June 1844 and the short branch line from Baden-Oos to Baden-Baden on 27 July 1845. On 11 February 1872 the bridge over the Rhine between Weil am Rhein and St. Louis was opened and the opening of the connection from Müllheim to Mulhouse on 6 February 1878 provided a further connections to Alsace. Connections to Switzerland were provided with the opening of the Rhine Bridge at Waldshut in 1859, the Konstanz–Kreuzlingen link in 1871 and the Basel Connecting Line, connecting the Baden station in Basel with the Basel central station in 1873.