After World War II, the city of Munich came to feature amenities that used to be reserved for Berlin and Hamburg before. In 1957, it became Germany's third million-denizen city and in the wake of the 1972 Olympic Games, it also became Germany's third city to feature an U-Bahn (by 1971) and an S-Bahn (by 1972). The U-Bahn was originally built to the same standard as the Nuremberg U Bahn and rolling stock was lended back and forth between the two systems (Munich took Nuremberg rolling stock for the 1972 Olympics, for instance) but newer generations of rolling stock no longer possess this ability.
In essence, the Munich U-Bahn network is a secant-system with three trunk lines (Stammstrecken) all bifurcating into two lines each and three transfer stations between them. This makes for six general services U1 to U6 and two special services U7 and U8. At the beginning, new U-Bahn lines got numeral designations derived from tram services they were supposed to replace. What is now U2 used to be U8 and what became U4 was initially planned as U9 replacing lines 9, 19 and 29. With the completion of the single triangle, lines were re-designated for avoiding gaps in the numeration scheme.
- Stammstrecke 1 features lines U3 and U6, built in the wake of the 1972 Olympic Games. Northwest-southeast-running with interchanges at Hauptbahnhof and Sendlinger Tor.
- Stammstrecke 2 features lines U1 and U2. Extra lines U7 and U8 also run there. Northeast-southwest-running with interchanges at Odeonplatz, Marienplatz and Sendlinger Tor.
- Stammstrecke 3 features lines U4 and U5. East-West-running with interchanges at Odeonsplatz, Karlsplatz (Stachus) and Hauptbahnhof.
- Under construction is the "U9 Bypass" which shall relieve Stammstrecke 1 by connecting their ends through a new trunk line under Hauptbahnhof (Main Station).
In order to bundle through the various suburban rail services around Munich, a railway tunnel between Hauptbahnhofnote and Ostbahnhof was built in order to access central parts of the city directly to the suburbs, inaugurating the new S-Bahn network with its opening. The Munich network was the first of its kind to be built after World War IInote and sister systems were opened in Paris (RER A, 1977), Frankfurt am Main (1978), Stuttgart (1978) and most recently Leipzig (2013). Back in the days, that tunnel used to be called V-Bahn (short for Verbindungsbahn, connector railway) and is now called Stammstrecke. It's the busiest railway in Germany, so busy in fact that Munich plans to build a second Stammstrecke in parallel to the first one with fewer stops in order to provide more capacities to the whole network.
When the U-Bahn network was built, streetcars were considered to be an expiring operation by the 1970s, eventually to be dismissed by the 1990s when the core of the U-Bahn was supposed to be commissioned. This didn't happen, the tramway was saved as a complement to the other modes of public transport and experienced major renovations and selective extensions.