RER Fribourg introduced the S20 designation on 14 December 2014 for an hourly service between Fribourg/Freiburg and Neuchâtel.[5] In December 2017, the S21, previously a rush-hour service between Fribourg and Kerzers, was re-routed to run between Fribourg and Ins and rescheduled for hourly service on weekdays, increasing the service level between Fribourg and Ins to half-hourly on weekdays.[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]