I'm having difficulty finding if there is rail service from Tours (or Amboise or Blois) to Chartres. I would then like to go from Chartres to Versailles enroute to Paris. Does this plan seem logical and do-able? This is my first trip to France.

No direct trains between Loire and Chartres - quickest way TGV Tours-Paris then from same station (Montparnasse) train to Chartres. But you can also take local trains Tours-Le Mans and then Le Mans-Chartres...but schedules are rather sporadic so do research for this way. www.sncf.com for schedules.

Depending on the rest of your itinerary - it would probably be easier to plan a day trip to Chartres after you are staying in Paris.

As for Chartres-Paris, yes, the regional trains between do stop at Versailles, but at Versailles-Chantiers. It's about a 15-minute walk to the chateau, or you can take a local bus.

There may be direct bus service between Tours/Orleans and Chartres.

There is direct Orleans-Chartres Trans-Beauce (name may have changed) bus service several times a day - but getting to Orleans from your towns then a slow bus would take much longer.
From Tours you can take a series of local trains and change a few times to Chartres but this is possible only at inconvenient times - mainly for schoolkids.
Yes better of day tripping to Chartres from Paris. And Versailles Chantiers station has no left luggage so you'd have your bags with you coming from Chartres.

Many thanks to everyone for your tips! Looks like the most efficient way is for me to go in to Paris and then do a side trip out to Chartres and Versailles. Was trying to catch these on my way IN since after Paris, I will be heading towards Lyon.

Versailles & Chartres in the same day. Possible:
Take as early a train as possible to Versailles via RER C - takes you to within few blocks of the palace. If you have a museum pass you can sidestep ticket lines - but if go early enough you won't have a long wait.

Be sure to walk around the gardens and park (come on weekends and you'll see the famous water fountains splishing and splashing to the sound of classical music piped around the gardens) - palace closed Mon

Then walk the mile or so to Versailles-Chantiers, the mainline station in town to hop hourly trains to Chartres - about 35 mins i think.

Train at Chartres dumps you off within eyeshot of the famous cathedral on its hill.

Return to Paris-Montparnasse station by mainline train from Chartres.

Chartres cathedral is open to around 7pm at least i think so do more time-consuming Versailles first.

And around sunset the light flooding in thru the cathedral's famous rose windows is surrealistic.

Take at least a short walk around Chartres lovely town centre by the cathedral. Pick up town walking tour and Walkman tour at tourist office between train station and cathedral.

Enjoy your day to Versailles and Chartres, easily combined IMO.

Manay thanks for all the suggestions, PalenQ! I will look forward to viewing the rose window at sunset. And hope my new digital camera gives me some keepers!