Upon the timetable change on 14 December 2008, an S-Bahn numbering system was introduced for regional rail services in Aargau.

[3] The system is supported by regional trains of Zürich S-Bahn, with railway stations in the eastern part of the canton being only served by the latter.

Since 2022, the section between Killwangen-Spreitenbach and Zürich Altstetten is additionally supported by tram line 20 (Limmattalbahn).

Stations in the northwest of the canton of Argau are served by the S1 of Basel S-Bahn (Frick/Laufenburg–Basel SBB).

With the December 2021 timetable change, Swiss Federal Railways applied "S" designations to three Regio services in the canton of Solothurn: These were numbered S20–S22 to avoid conflicts with the Aargau S-Bahn and are not part of the network.