The S21 is a regional railway service of the Zürich S-Bahn of the Zürcher Verkehrsverbund (ZVV). The service operates at peak hours between Regensdorf-Watt and Zürich Hauptbahnhof over the Wettingen–Effretikon railway, supplementing the regular S6 service, which continues west to Baden.

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S21
Trainset at Zürich HB
Overview
Locale	Zürich, Switzerland
Termini	Zürich Hauptbahnhof Regensdorf-Watt
Stations	5
Service
Type	S-Bahn
System	Zürich S-Bahn
Operator(s)	Zürcher Verkehrsverbund (ZVV)
Rolling stock	Re 450 and double-decker coaches Re 420 (LION) locomotives and double-decker coaches
History
Opened	13 December 2015 (2015-12-13)
Technical
Track gauge	1,435 mm (4 ft 8+1⁄2 in) standard gauge
map_name Legend time (h) S6 to Buchs-Dällikon 0:00 Regensdorf-Watt S6 0:03 Zürich Affoltern (limited service) S6 S2 S16 S24 to Zürich Airport S7 to Opfikon S3 S9 S15 to Oberglatt S8 S14 S19 to Wallisellen 0:12 Zürich Oerlikon S2 S8 S14 S19 to Zürich HB S24 to Zürich Wipkingen Käferberg Tunnel (2.12 km) S5 S11 S12 S42 to Zürich Altstetten 0:16 Zürich Hardbrücke S3 S5 S6 S7 S9 S11 S12 S15 S16 S24 to Zürich Wipkingen S14 S19 to Zürich Altstetten S2 S8 S24 S25 to Zürich Wiedikon S4 S10 from Zürich HB SZU to Zürich Selnau 0:22 Zürich HB[lower-alpha 1] S2 S8 S14 S19 to Zürich Oerlikon S3 S5 S6 S7 S9 S11 S12 S15 S16 S20 S23 to Zürich Stadelhofen [1][2] This diagram: view talk edit Zürich HB tracks (Gleis) from left to right: 21–22 (SZU) / 31–34 / 3–18 (surface) / 41–44.

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At Zürich HB, trains of the S21 service usually depart from ground-level track (Gleis) 18.

• S21 Regensdorf-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]

• Zürich Hauptbahnhof
• Zürich Hardbrücke
• Zürich Oerlikon
• Zürich Affoltern
• Regensdorf-Watt

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]

As of the December 2022 timetable change,^[update] most services are operated by Re 450 class locomotives pushing or pulling double-deck passenger carriages.^[6]