S-Bahn f (genitive S-Bahn, plural S-Bahnen)
This is not(!) Strassenbahn, which is a slow train for city centers. Sometimes you need to take the S-Bahn to get to the next city and then switch to the local, slower, Strassenbahn to get to your final destination. For people from California: S-Bahn is more like BART, while Strassenbahn is like a cable car.