Where to Eat at Charles de Gaulle Airport

You can look on. the airport website looking under the terminal you will arrive in and then search for restaurants. It is VERY slim picking landside and you won't have access airside. I came in to 2A so I looked there -- apparently landside there is a place called Frenchy's and a MacDonalds --

I'm a fan of the offerings at PAUL, and I have eaten at the one in terminal 2 at CDG. Nothing fancy, but simple and tasty. Depending on the timing, I might take my selections with me to eat on the train.

I like Paul too.

Reminds me of the various ‘bread cafes’ in the US, but I’m guessing with better bread! It would serve the purpose but I’m hesitant to bring food on the train. How common is it for people to eat on trains?

Much better than Panera, and eating on the train is very common. Drinking wine or beer with your train meal is equally common. TGV trains have seatback tray tables.

http://seat61.com/trains-and-routes/tgv.htm

Taking a picnic on the train is very common.

Very, very common to bring food and drink (including alcohol, it's allowed) on trains.
This said, if you feel short of time or if you are not inspired, TGV trains (not Ouigo) have a cafe on board. The food is very simple and a bit overpriced (small servings!) but much better than it was 10-15 years ago. It won't be as good as Paul, but better than McDonald's! And I never tire of the novelty of 300 km/h breakfasts, lunches and dinners.

Anyone have any experience with Cafe Eiffel?

Paul, as others have said, will no doubt be the easiest. It's not bad. Though about a decade ago it was judged that they couldn't call themselves a "boulangerie" because their products are industrial.

Are you arriving at 2E from your international flight? As you come out from Baggage Claim and thru Customs you empty right into a waiting area with a M&S Simply Food outlet which has pretty good premade sandwiches, salads, snack chips, drinks, etc.

If this is an international arrival you will not go by any food from your gate to Immigration to baggage to customs to exit to the non-secure area.

Are you getting on the TGV there at CDG? There is another Paul right by the TGV tracks, a Brioche Doree (top floor of the train station, though I forget which side), and several other options in the train station itself.

Yes, we are getting on the train at CDG. It would be very convenient to get food near the train, thanks!