Forget about the train. It is long, not cheap, boiling hot inside at this season, and there are not so many of them away from the commuting hours.
My very best recomandation, which I give to all of my guests here, is to drive the A6 up to "porte d'Orleans", right at the end of that motorway, where there is a 24/7 undergound car park in which you have a direct access to the metro station.
Lots of advantages, especially that you don't have to check out on the time tables to come back in the evening.
You are more than welcome to send me a message for more info and details if interested.