Chartres and Versailles in 1 day trip?

It is very doable.

The train to Chartres is only an hour. Seeing the cathedral only takes an hour (I assume there is nothing else there you want to see). The train from there to Versailles is only an hour. So assuming you leave Paris by 8-9 AM you can be in Versailles by around noon or just a little after.

If you wish to avoid the worst of the tourist crush at Versailles, you should go there first thing in the morning instead. Then, when you have seen it all and are completely exhausted at around 3-4 PM, you can sit on the train for an hour and recover a bit on your way to Chartres. Then see the cathedral and have dinner there (relaxing some more), then train into Paris.

Randy has the order absolutely correct. Better grub choices and better prices in Chartres. Personally, I'd only allow an hour for Versailles and spend it drinking coffee and looking in the gate with binnoculars.

Here's how I would do it. Go to Versailles first and be there a little before the chateau opens to beat the crowds. Buy the fixings for a picnic lunch and eat it on the one-hour train ride to Chartres. Visit Chartres in the afternoon. Either have dinner in Chartres or wait until you return to Paris.

Thank you all for your input. I hadn't thought of going to Versailles first...thinking it was better to go to the farthest point first. Going to Versailles first makes much better sense since we will be spending the better part of the day there. I think all there is to see in Chartres is the cathedral, is that correct? Dinner in Chartres and an uninterrupted train ride back to Paris sounds good. Thanks for the advice...now to check train schedules!