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]