BAM subsidiary Wayss & Freytag Ingenieurbau has been carrying out the tunnelling as part of a joint venture working for client DBPSU.
The small group of people invited to witness the breakthrough included Baden-Württemberg's transport minister Winfried Hermann, DB infrastructure director Ronald Pofalla and tunnel sponsor Andrea Klöber.
The breakthrough, which took place at a depth of about 13m, completes the drive for the 3.5km-long tunnel. Some further work continue until about mid-December.
The new tunnel will connect the Mittnachtstraße S-Bahn station via the new Neckar Bridge with the existing route from Stuttgart Central Station in the direction of Cannstatt from 2025.