The dispute started between Kettenbrückengasse and Pilgramgasse stations, at 6.30am, and rapidly escalated - with one of the men spraying pepper spray on the other, and throughout the train carriage.
Someone pulled the emergency brake, and the driver was alerted. The train was evacuated at Pilgramgasse. The APA news agency reports that an ambulance took two pepper spray victims to hospital, but that the man who had sprayed it fled before he could be arrested.
The Wiener Linien said that the U4 line was closed between Margaretengürtel and Karlsplatz for about 25 minutes, during which time a rail replacement bus service was provided. Many commuters complained that they were late for work.