Side trip from Chartres to Le Mans

Which museum? The 24 hours museum?
In any case: while I have never made the trip from Chartres, I have been to Le Mans and it is a lovely place for a day trip. There is much more there than the 24 hours; the old town is really beautiful and the cathedral is quite impressive, one of my favorites in France.
Trains from Chartres are relatively frequent and easy, takes about 1.5 hours one way if memory serves. No reservation needed. Once in Le Mans, the tram service is efficient to go to the old town on one side, less so for the 24 hours museum (the stop is a long walk from the museum) although there are buses as well.

Balso is right. Le Mans is a fascinating destination and the old city is very compact. The remaining Roman walls are amazing.

I can't help with the train, but I want to say that both cities have nighttime illuminations. Chartres's are magnificent. Le Mans has fewer and they aren't as impressive. So if you are in Chartres, be sure to stay over at least one night.

Thanks! This is helpful.