The S21 is a railway service of RER Fribourg that provides hourly service between Ins and Romont, in the Swiss cantons of Vaud and Freiburg, respectively. Transports publics Fribourgeois operates the service.

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S21
Overview
First service	14 December 2014 (2014-12-14)
Current operator(s)	Transports publics Fribourgeois
Route
Termini	Ins Romont
Stops	17
Distance travelled	51.7 kilometres (32.1 mi)
Average journey time	1 hour 22 minutes
Service frequency	Hourly
Line(s) used	Fribourg–Ins line Lausanne–Bern line
Route map Legend S5 S20 to Neuchâtel 32.2 Ins S5 S20 S52 Regional services to Biel/Bienne S5 S52 to Kerzers 27.8 Sugiez S20 S5 R9 to Kerzers 24.2 Muntelier-Löwenberg S5 R9 22.2 Murten/Morat S5 R9 S20 S5 R9 to Payerne 20.6 Münchenwiler-Courgevaux S20 16.9 Cressier FR S20 12.4 Courtepin S20 9.2 Pensier S20 5.8 Belfaux-Village S20 S30 to Payerne 4.0 Givisiez S20 S30 S1 to Bern 0.0 66.0 Fribourg/Freiburg S1 62.1 Villars-sur-Glâne S20 60.4 Matran S20 57.5 Rosé S20 55.4 Neyruz S20 52.4 Cottens S20 49.6 Chénens S20 45.0 Villaz-St-Pierre S20 40.2 Romont RE2 RE3 R6 S20 R6 to Palézieux RE2 RE3 to Bulle [1][2] This diagram: view talk edit

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The S21 runs hourly between Ins and Romont. It uses the western end of the Bern–Neuchâtel line, the entirety of the Fribourg–Ins line, and a portion of the Lausanne–Bern line. It is paired with the S20 between Ins and Fribourg/Freiburg, and between Ins and Romont on weekdays, providing half-hourly service.^[3][4]

RER Fribourg introduced the S21 designation on 14 December 2014 for a rush-hour service between Fribourg and Kerzers.^[5] In December 2017 it was changed to an hourly service on weekdays between Fribourg and Ins, paired with the S20 and together providing half-hourly service.^[6] With the December 2021 timetable change, both the S20 and S21 were extended south from Fribourg to Romont, replacing the S40.^[7] The S21 began running on weekends with this change, while the S20 operated between Fribourg and Romont on weekdays only.^[8]