According to the Transilien departure board (https://www.transilien.com/fr/horaires/prochains-departs/?uicDeparture=8754519&departure=Br%C3%A9tigny&uicArrival=8754524&arrival=Juvisy), northbound through RER C services are normally scheduled to depart from platform 4 (track 2 bis).

Hello, according to Transilien (https://www.transilien.com/fr/horaires/prochains-departs/?uicDeparture=8754513&departure=%C3%89tampes&uicArrival=8754524&arrival=Juvisy), all northbound RER C services from Saint-Martin-d'Étampes normally depart from Platform 4 (track 2 bis) under regular operations.