S4 (St. Gallen S-Bahn)

S4
S4
	S4 trainset near Sevelen
Overview
First service	15 December 2013
Current operator(s)	Südostbahn
Route
Termini	Sargans; Rapperswil
Stops	28
Distance travelled	141.9 kilometres (88.2 mi)
Average journey time	2 hours 23 minutes
Service frequency	Hourly
Line(s) used	Chur–Rorschach railway line; Rorschach–St. Gallen railway line; Bodensee–Toggenburg railway line; Uznach–Wattwil railway line; Rapperswil–Ziegelbrücke railway line;
Route map
Route map
	46.3
	34.4
	20.0
	53.2
	St. Gallen
	Appenzell Ausserrhoden
	Appenzell Ausserrhoden
	St. Gallen
	21.5
	80.5
	VAE S1 S2 S5 S20
	S21 S22 S81 S82

The S4 is a railway service of the St. Gallen S-Bahn that provides hourly service between Sargans and Rapperswil via St. Gallen, connecting stations in the cantons of St. Gallen and Appenzell Ausserrhoden. The section near Lake Constance (Bodensee) is also part of the Bodensee S-Bahn. Südostbahn (SOB), a private company primarily owned by the federal government and the canton of St. Gallen, operates the service.

Between 2013–2023 it operated over a circular route (German: Ringzug), also running over the Ziegelbrücke–Sargans railway line south of the Walensee.