Booking ahead guarantees you the best fares on TGV high-speed trains (long distance trips), but Chartres is so close to Paris that there is no difference in price. So you can buy your ticket once you are there. The fare is always the same: 12.90 €.

There are trains every 40 minutes during rush hour then every hour. Here is how to download the schedule:

Chartres is only an hour away from Paris.