The name was soon adopted for Hamburg's city-owned street-independent mass transit lines. (There also is a Reichsbahn-owned and operated S-Bahn in Hamburg).
Both the Hamburg and Berlin U-Bahn fit the criteria for a metro. (Fast, electrically operated trains, completely independent from other traffic).
As the Post-World War II rebuilding had lead to some wealth and prosperity in West Germany, the German car fanaticism motivated many larger city councils to plan the replacement of the "car traffic obstructing" tramways with U-Bahn-Systems and additional Buslines. Nürnberg(Nuremberg) and München (Munich) decided for a full subway, (like those in Berlin and Hamburg) independent from their existing tramways (The tramways were meant to be abandoned in the 1990s). Stuttgart, Frankfurt, Köln (Cologne), Bonn, Düsseldorf, Duisburg, Bochum, Essen, Dortmund, Hannover and Bielefeld (list complete?) started to built tunnels for their existing tramway cars, rebuilding existing tramwaylines under ground. Those systems of classical tramways routed through tunnels in downtown areas do not meet the criteria for a metro. Nonetheless, they are usually referred to as U-Bahn. Officially, they are called Stadtbahn[?] (City Railways) or U-/Stadtbahn[?]. (In German urban transport newsgroups, recurring disputes appear if any of the aforementioned cities' (especially Frankfurt's) mass transport system can be called U-Bahn (implicitly stating it meets the criteria specified in metro), or if it has to be referred to as Stadtbahn[?].)
During the 1990s (when, according to original planning, the tramways of Nürnberg and München were scheduled to disappear), a reorientation process set in. Shortage of money, increased rider numbers and the insight that larger streets only attract even more cars slowed the building of subway lines and let to a renaissance of the tramways in those cities that had forgotten them. In Nürnberg and München, after 30 years new rolling stock was purchased, existing lines were were modernized and new ones were built, leading to new integrated traffic concepts.
wikipedia.org dumped 2003-03-17 with terodump