The word Stammstrecke (in English literally "trunk route" or "trunk stretch") is used to refer to the stretch of S-Bahn between Donnersbergerbrücke and Ostbahnhof. All eight S-Bahn lines (or however many there are these days) share this stretch of track.
The Stammstrecke is, as the name suggests, a bit of a bottleneck in the Munich S-Bahn system. If one train, or set of points, or signals on this stretch fails, the entire network is subject to delays.
There are plans to build a second Stammstrecke - this should create some redundancy in the system and ease congestion. The plans were published in September 2005. It will be at least a year or two before the building work is complete.
See also: