S21 (RER Fribourg)

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

Operations
The S21 runs hourly between Ins and Romont FR. 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 FR on weekdays, providing half-hourly service.

History
RER Fribourg introduced the S21 designation on 14 December 2014 for a rush-hour service between Fribourg and Kerzers. 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. With the December 2021 timetable change, both the S20 and S21 were extended south from Fribourg to Romont, replacing the S40. The S21 began running on weekends with this change, while the S20 operated between Fribourg and Romont on weekdays only.