'Second Trunk Line') is an under-construction double track commuter rail tunnel in the Bavarian capital of Munich, Germany.

Nearly all lines of the Munich S-Bahn use the Stammstrecke through the city centre in the underground, creating a bottle-neck responsible for long and, consequently, increasingly frequent delays from even the smallest disruptions.

The route has been upgraded to its limits to enable shorter headways including by implementing the Spanish solution at busy stations, automatically opening all doors as soon as the train stops (as opposed to the more usual opening of train doors only when a button is pushed) and upgrading signaling.

After years of discussions and studies into different route propositions, a second tunnel through the city centre has been already approved with the funding of €3.85 billion and the completion date of 2026.

With focus on express service and shorter travel time, the second tunnel will bypass six current stations between Laim and Ostbahnhof.