We will be visiting Paris this coming May. I am definitely going to purchase our entrance tickets to the palace well in advance. My question is do we need to purchase our train tickets ( we are 4 adults) well in advance as well? Or can we just show up at the train station on the day we want to visit? Thanks.
no need to purchase train tickets in advance.
No, these are suburban trains without reserved seats, and don't need advance booking.
Your Paris Visite or Navigo pass will also work on the RER C.
Versailles has two train/Metro/RER stations. You want the one that is closer to the Chateau, Versailles-Rive Gauche
https://parisbytrain.com/paris-to-chateau-versailles-by-train/
Enjoy your trip.