Danube
The Danube flows through many cities, including four national capitals (shown below in bold), more than any other river in the world. Ordered from the source to the mouth they are:
The Danube is navigable by ocean ships from the Black Sea to Brăila in Romania and by river ships to Kelheim, Bavaria, Germany; smaller craft can navigate further upstream to Ulm, Württemberg, Germany. About 60 of its tributaries are also navigable.
Danube Valley Railway (Baden-Württemberg)
The Danube Valley Railway (German: Donautalbahn or Donaubahn) in Baden-Württemberg in south-western Germany is a 133.8-kilometre-long railway running from the city of Ulm to Immendingen, which is largely single-tracked and for the most part not electrified. The line is famous especially for its charming course through the Upper Danube Nature Park ("Naturpark Obere Donau"), and is particularly attractive to bicycle tourists. The Royal Württemberg State Railways and the Grand Duchy of Baden State Railways built the line as part of the railway projects undertaken between 1865 and 1890. The construction of the section between Tuttlingen and Inzigkofen was pushed through by the German general staff, for whom the Danube Valley Railway was seen as a strategic railway in case of another war with France. Since 1901, the Danube Valley Railway, together with the Höllentalbahn (Schwarzwald), form part of the pan-regional railway link from Ulm to Freiburg im Breisgau.