Train Travel between CDG Airport and Chartes

I am sure that they have, but it involves getting to Montparnasse station. You can either take the RER to Denfert-Rochereau, then the No 6 Metro to Montparnasse, or take a fixed price taxi from CDG to Montparnasse.

PS, Another way, which perhaps is your point, is to take the RER B to Massy- Palaiseau, then change to the RER C to Versailles Chartiers, then change to the TER to Chartres. This does not show up on rail planners because the RER B is operated by the Paris Metro system and the RER C and TERs are operated by SNCF. Whether this is better than the RER B, Metro 6 and TER from Montparnasse I know not.

PPS. The IDS trip planner: https://www.iledefrance-mobilites.fr/en, Recommends the Denfert-Rochereau connection as the fastest in about 2 hours. At 18 minutes slower, but less walking is the change from the RER B at Saint Michel-Notre Dame to the RER C to Versailles Chantiers to get the TER to Chartres.

Going via Massy Palaiseau does not make much sense as it is much longer and the V train from Massy to Versailles is infrequent. Likewise, I do not recommend taking the C from St Michel to Versailles Chantiers either , since it is also much longer and less frequent. This leaves the RER B to Denfert-Rochereau where one transfers to metro line 6 to Montparnasse.

If the stairs in the metro are troublesome, you can exit the RER B at Denfert via elevator and take a cab for the remaining 10€ trip to Montparnasse.

I'd say take a cab to la Gare Montparnasse and then take the TER to ChartRes.