Train from Paris to Chartres on Sunday morning

I am showing a train every hour. How frequently do you want them to run? Coming back is about the same. It's an easy one hour trip.

I saw a post that said that they didn’t run regularly so I was concerned.

Download DB Navigator, RATP, and SNCF to your phone or tablet and you can check all the rail connections in France. They are lifesavers in trip planning and traveling.

I once took an early morning Sunday train to Chartres, saw the cathedral, attended a Tridentine Mass (in Latin) in the cathedral, returned to Paris, ate lunch near the Luxembourg Gardens, went to the Catacombs after lunch and then ended the day with a solemn high mass at Notre Dame Cathedral, then ate dinner at a creperie on one of the islands, washing everything down with cold cider. What a day!

The website has changed to "OUI.sncf" so you might try that.
You should probably purchase tickets as soon as you can - there are options for earlier travel and later travel on the calendar.
OUI.sncf [Voyages-sncf.com]