The S12 is a regional railway line of the S-Bahn Zürich on the Zürcher Verkehrsverbund (ZVV), Zürich transportation network, and is one of the network's lines connecting the cantons of Zürich and Aargau.
Line S12 commences at Brugg station, in the canton of Aargau, and follows the Olten to Zurich line as far as the approaches to Zurich Hauptbahnhof station. Passing through the lower level platforms at this station, the line then passes through the Hirschengraben and Zürichberg tunnels, and Stettbach station, before joining the Zurich to Winterthur line.
The S12 follows this line as far as Winterthur Hauptbahnhof station, running non-stop between Stettbach and Winterthur. At Winterthur, alternate trains take different routes, running either over the Tösstalbahn as far as Winterthur Seen station, or over the Winterthur to Etzwilen line as far as Seuzach station.