The S21 is a regional railway service of the Zürich S-Bahn of the Zürcher Verkehrsverbund (ZVV). The service operates at peak hours between and over the WettingenâÂÂEffretikon railway, supplementing the regular service, which continues west to .
At , trains of the S21 service usually depart from ground-level track () 18.
The service operates on weekdays during peak hours, roughly from 6:30ÃÂ am to 8:30ÃÂ am and from 4:30ÃÂ pm to 7ÃÂ 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 (or arrive at) 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:
The S21 designation was previously used for service from Zürich to over the ThalwilâÂÂArth-Goldau railway; this is now provided by the . The current S21 began running with the December 2015 timetable change.
most services are operated by Re 450 class locomotives pushing or pulling double-deck passenger carriages.