We will take a day trip from Paris to Chartre by train, then fly from Orly to Marseilles that same evening. Where can we store our luggage so that we don't have to take it sightseeing in Chartre?
Trains from Paris to Chartres depart from Gare Montparnasse. The station has left luggage facilities. I suggest that you store your luggage there and pick it up when you return from Chartres.
This is a question we see often on this page. It becomes more difficult daily to leave luggage. Many places have removed lockers as a security measure. You never know the nearest place to store luggage. So you have to ask.
I suggest your ask someone wherever you are. It may be in the RR station or air terminal. Sometimes it will be in the nearest hotel.