Bringing food & drink on a RER train

Certainly. You'd have to, actually, since they have no dining cars.

Hopefully, you ride the metro/rer around town beforehand so you'll be able to figure out how you want to set thins up.

I don't know about Gare D'Austerlitz, but the RER stations at Gare Du Nord have coffee/snack bars right on the platform so you can get your jo to go on the way to the airport. Every 8 minutes 500 new potential customers.

I had a bottle of water on the RER with no issues.

When you're heading to Versailles, the RER C line forks out, so make sure you're on the Versailles Rive Gauche line or you will need to change at the station Champs de Mars/Tour Eiffel to catch the Versailles Rive Gauche line.

While Seattle's metro buses do have rules against food and drink, I've rarely seen the same on any form of public transport in Europe.

Once in a blue moon nowadays you see someone discretely chomping on something still wrapped in paper in the metro or RER car; you don't see people drinking inside the cars. It used to be that no one ever ate or drank anything in the metro or RER, maybe because it's so dirty and people are crowded together. It's pretty tacky. People don't walk around with coffee cups and drink cups the way they do in the States. Plastic bottles, yes. You might want to snack while you're walking outside on your way to the Palace.

After we visited Versailles we got McDonald's and ate on the RER back to Paris. The train was so crowded that we had to stand up, but we were really hungry and ate anyway. No one said anything to us.

No one will say anything (you might get a few looks of disdain), but it's not common practice to eat and drink on public transportation in Europe (trains excluded). They simply don't walk around eating and drinking on the go the way we do.

Note - a bottle of water is a totally different story.

OK, thanks for the responses!