The S-Bahn and U-Bahn are the two primary rail systems in German cities, and while they often share stations, they serve different functions. The U-Bahn (Untergrundbahn) is the "underground" or metro system; it is primarily urban, with frequent stops located close together within the city center. It usually operates on its own dedicated tracks, often entirely underground. The S-Bahn (Stadtschnellbahn) is a suburban commuter rail system. It covers much larger distances, connecting the city center to outlying suburbs and neighboring towns. S-Bahn trains are generally larger, run less frequently than the U-Bahn, and often share tracks with regional or national heavy rail. Visually, U-Bahn stations are marked with a blue "U," while S-Bahn stations use a green "S." In short, use the U-Bahn for hopping between city neighborhoods and the S-Bahn if you need to travel to the airport, a distant suburb, or a nearby city. Both systems are typically integrated into a single ticketing system, so you can transfer between them on one ticket.