At Zürich HB, trains of the S21 service usually depart from ground-level track (Gleis) 18.
Route
S21Regensdorf-Watt–Hardbrücke–Zürich HB Operates only during rush hour; non-stop between Oerlikon and Regensdorf-Watt; morning services to Zürich HB and evening services to Regensdorf-Wall also stop in Zürich Affoltern.
The service operates on weekdays during peak hours; there are a total of eight trains in each direction per day. Inbound trains in the morning and outbound trains in the evening bypass Zürich Seebach. Trains usually depart from (or arrive at) one of the surface platforms of Zürich HB. The S21 calls at the following stations:[4]
The S21 designation was previously used for service from Zürich to Zug over the Thalwil–Arth-Goldau railway; this is now provided by the S24. The current S21 began running with the December 2015 timetable change.[5]