S21 (ZVV)

The S21 is a regional railway service of the Zürich S-Bahn of the Zürcher Verkehrsverbund. The service operates at peak hours between and over the Wettingen–Effretikon railway, supplementing the regular service, which continues west to.
At, trains of the S21 service usually depart from ground-level track 18.

Route

The service operates on weekdays during peak hours, roughly from 6:30 am to 8:30 am and from 4:30 pm to 7 pm; there are a total of eight trains in each direction per day. In the morning only inbound trains stop at, in the evening only outbound trains. Trains usually depart from one of the surface platforms of. By being overlapped with, services between and operate with a headway of roughly 10 or 20 minutes respectively. The S21 calls at the following stations:
*

History

The S21 designation was previously used for service from Zürich to over the Thalwil–Arth-Goldau railway; this is now provided by the. The current S21 began running with the December 2015 timetable change.

Rolling stock

most services are operated by Re 450 class locomotives pushing or pulling double-deck passenger carriages.