The Vienna U-Bahn (German: U-Bahn Wien), where U-Bahn is an abbreviation of the German term Untergrundbahn (English: underground railway), is a rapid transit system serving Vienna, Austria.With the September 2017 opening of the 4.6 kilometers (2.9 mi), five-station extension of the U1 line, the five-line U-Bahn network consists of 83.1 kilometers (51.6 mi) of route, serving 109 stations.