Advice on RER tickets at CDG

Buy when you arrive. It's not a problem. You can buy from a live ticket agent with a small wait in line.

If you can buy last minute - and pay last-min fares - see if you can make the direct train from CDG. You may be able to buy them with the SNCF app on your mobile phone if that's possible.

If not, try buying RER tickets from a machine if there's a long wait in line.

You can't buy RER tickets in advance but you also don't want to wait in line; that line can easily be an hour; been there done that. Buy them from a machine at the RER station. I don't know your passport but even US chip/sig cards will work in those machines for small purchases. The tickets are 10.30. And yes you are right that you should not book a train 90 minutes after arrival. You should leave 4 hours between cDG arrival and Gare Montparnasse departure. You might have time for lunch and coffee at the station, or if your plane is late and you are in a two hour immigration line you might barely make it.

What Jane said.
I have waited up to and beyond two hours, just to get to the passport control desk at CDG.
A woman in the line fainted as we were all waiting.
(Find a bathroom upon landing just in case; as you can't leave the lineup once you're in it!)
I have also waited nearly an hour in line to get to the machines selling the RER tickets, never mind the actual desk with a person on it.
I was in Paris this time last year, and same time in 2016, and the place was mobbed; so who knows what the crowds will be like in May.
Leave at least 4 hours after landing till you plan to be at G. Montparnasse.

I think you have a good chance of catching that train to Tours. Wait until you get to CDG and decide depending on when you get to the station.

I have a few questions about RER tickets.

1. Are the RER ticket machines whether at CDG or Gare do Nord only in
   French?

2. Are the RER B tickets for a specific train? Specific date?

3. Can you buy roundtrip tickets between CDG and Paris? If so, must
   you commit to a date and time for the return ticket to CDG?

When we were in CDG in 2015, the RER ticket machines only took coins or chip/pin cards. Is that still true?

Thanks all for the input. I was hoping there was some way to buy the RER tickets online so we could avoid the lines at the station.
BT

The machines will take Chip/sig cards -- when it asks for the PIN you just ignore and proceed. For small sums like a couple 10.30 tickets they will work. They don't take bills though in my experience.

The machines have an English option.

RER tickets are point to point; in this case CDG/Paris (will take you anywhere in Paris) They are good forever until used not date specific. There are no round trip tickets. You buy two; they work both ways so from any metro station in Paris to CDG. That is pretty much the way all train tickets in France work i.e. one way tickets.

I don't believe RER tickets can be bought in advance. The last time for me at CDG was in 2015. I bought the RER ticket to Gare du Nord at a staffed ticket counter, not much of a wait, which didn't matter anyway since I was not connecting.

If anyone is interested in more "how to's", several videos on this subject on you tube.

The first time I took RER B into Paris from CDG. I arrived at the ticket machines. There was no line at the ticket window but I prefer machines anyway. I believe it offers an English button at the start of the transaction. I can, at least, read French so I don't generally use the translation.

First attempt, it says the transaction is declined. No problem, I'll try my debit card rather than my credit card. Same result. I decide I better use the ticket window. I step away from the machine, look at the ticket window and now there is a long line plus a short line for the machines (and now "helpful" people hovering over tourists). I get back in line for the machines and decide to try a different machine than I used the first time. I use my credit card and it spits out my ticket.

I've had a similar experience with so many RER and metro ticket machines in Paris, I now have a theory that the machine times out before my transaction is approved. For some reason, a second attempt seems to work as often as not. If a second attempt doesn't work, I try a different machine. The particular machine may not be working. I've also had second attempts that say the transaction wasn't completed but my ticket spits out. So check there before you leave the machine. I've also had gates that didn't open when I put my ticket through. When that happens, use a different gate (or are you using a spent ticket?) At toll booths, I often have to use every card I pack, twice, before something goes through. It seems like each trip there is one (only one) toll booth where nothing but cash works - so keep some cash with you just in case.

Toll booths on the highways are run by different companies in different parts of the country (the joys of 'privatization') Some of them take US cards with no problem; some of them don't. You never know till you try and some tole sites don't have machines that take money and such sites also don't have manned booths. There is sometimes a guy on the intercom who can rescue you if your cards won't work.

The price for the RER ticket in 2015 was 10 Euro, stated on the ticket.

what you should do is buy them from one of the TI stands at CDG. http://en.visitparisregion.com/practical-info/tourist-information-desks-100180.html This is like a hidden secret. The staff speaks english, they accept American credit cards, there is seldom a line, and they sell RER/metro tickets.

The line at the RER station takes forever, and the RER machines don't accept many US credit cards.

Getting a RER ticket is a matter of luck, far more of a chance that you'll be waiting in line for Metro tickets (or a much longer line) than you would for RER tickets. I've had much better luck in terms of short lines or non-existing ones in front of a RER counter than one for Metro tickets. Of course, that depends at which station you are buying the Metro tickets.

Regarding toll booths -- make sure you have some cash in small bills for the one that doesn't take your card. Or carry a bag for the change you will get (all coins) if all you have is a €50 note for a €4 toll. (Voice of experience!)