R4 (RER Vaud)

The R4 is a railway service of RER Vaud that provides hourly service between Le Brassus and Aigle in the Swiss canton of Vaud. A limited number of trains continue from Aigle to St-Maurice. On weekdays, a section of the train operates from Le Day to Vallorbe. Swiss Federal Railways, the national railway company of Switzerland, operates the service. The service was previously known as the S4.

Operations
The R4 operates every hour between Le Brassus and Aigle, using the western end of the Simplon line. The R4 makes a limited number of stops between Lausanne and Aigle. The R4 is paired with the R3 between Le Day and Aigle, providing half-hourly service. Both the R3 and R4 provide limited service beyond Aigle to St-Maurice. On weekdays, a section of the R4 operates between Le Day and Vallorbe.

History
The "first" S4 was one of the six original lines of the RER Vaud, then called the Vaud Express Network (Réseau express vaudois, REV), when that system was established in December 2004. It ran hourly between Morges and Palézieux. SBB extended the S4 west from Morges to Allaman with the December 2011 timetable change, making local stops. Rush-hour service to Romont began on 10 December 2017. In December 2021, SBB increased the number of rush-hour round-trips between Allaman and Romont from three to six.

The RER Vaud lines were substantially reorganized for the December 2022 timetable change. The "new" S4 was a combination of the former S2 and S22, making local stops on the Simplon line between Vallorbe and Lausanne, and limited stops between Lausanne and Aigle, with limited service from Aigle to St-Maurice. On 10 December 2023, all RER Vaud lines were renamed as "R" and a number, instead of "S."