Hi mrfurrier!
The S-Bahn mod isn't available via the filebase. You have to download it with eis_os' Mod Manager which is available here.
You install the Mod Manager by placing the .gs file in your addons folder. (C:/Program Files/Cities in Motion/addons or C:/Program Files/Steam/SteamApps/common/Cities in Motion/addons if you run a Steam version)
Run the game, the installation routine of Mod Manager should start. When you are done, reactivate your mods via the Mod Manager.
Add mediziners repository as a source in Mod Manager (it should be a predefined source), download S-Bahn mod, activate it, start a savegame and be happy...
I hope I could help you.
Also einfach den ModManager von eis_os runterladen, die .gs File in den ''addons'' Ordner in deinem Cities in Motion-Hauptordner packen. Falls nicht vorhanden, einfach erstellen -> ''addons''-so schreiben! CiM starten, installieren lassen. Dann einfach unter dem Reiter ''Install'' nach dem S-Bahn Mod suchen und runterladen, unter Mods/Addons aktivieren und: Tada, einach ein Spiel starten. Hoffe, ich konnte helfen