Some cities, most notably Frankfurt, Stuttgart, Hannover, Cologne, and cities in the Ruhr region, have relatively new light rail systems known as
a Stadtbahn. Generally, these systems function very much like a regular U-Bahn system (subway, coming next!) with wide-gauge tracks, longer trains, and high platforms.
However, while the Stadtbahn usually runs in extensive tunnel networks within the central city areas, it runs mostly overground outside of the central city. When running overground, the Stadtbahn runs almost entirely in exclusive rights-of-way, making them faster than streetcars. These systems generally serve the central city and the immediate vicinity. Service is frequent, usually 10-20 minutes during off-peak periods. Stadtbahn lines are typically numbered with a "U" followed by a number (e.g. U9).
Most Stadtbahn systems mark their stations with the standard "U" U-Bahn sign with the word "Stadtbahn" added below or across the "U".