U4 is the shortest line in Berlin’s U-Bahn system.
The line runs only 2.86 kilometres between Nollendorfplatz and Innsbrucker Platz in the district of Schöneberg. It has five stations, all within Berlin, making it dramatically shorter than the city’s other established U-Bahn routes. U4 is also notable because it has never been extended beyond its original alignment.
Schöneberg built the railway independently after private investors considered the project insufficiently profitable. Construction began in 1908, and the line opened on 1 December 1910. Operations were handed to the company that ran Berlin’s elevated railway, the predecessor of today’s BVG system.
The U4’s small scale can cause confusion with Berlin’s former U55 shuttle, which was also exceptionally short. U55 was temporary and ceased to exist as a separate line when it was connected to U5 in 2020. U4 remains a permanent line and retains the distinction of being Berlin’s shortest U-Bahn route.