Trains to Maintenon leave the station Paris-Montparnasse about once an hour, but not always on the same minute. The journey takes between 50 and 55 minutes, with stops in Versailles-Chantiers, Rambouillet and Epernon. After leaving Maintenon, the trains go on to Chartres.
In 2014 I paid € 9.70 for a one-way senior ticket departing in the “blue period” (départ en bleu). The French railway system SNCF differentiates between the “white period”, meaning the morning and evening rush hours, and the “blue period”, meaning the rest of the day and all day Saturday. Tickets for departures in the white period are somewhat more expensive.
This machine claims to be “simple and rapid” if you want to buy any train ticket without a seat reservation. I did indeed find it simple and rapid as I needed a ticket from Maintenon to Paris with about one minute to spare before the train arrived. After buying the ticket you have to “compost” it in the small yellow machine, which validates it for immediate use and invalidates it for any future use.
In the TER train (=Transport express régional) on the line from Paris-Montparnasse to Chartres via Maintenon.
My photos in this post are from 2014. The text was last revised in 2017.