CDG to Colmar

Two hours after landing, at least: taxiing, deplaning, long walk through corridors, immigration, luggage reclaim, customs, walk to station.
Immigration (passport control) is the big delay, customs is literally a 30 second walk through.

But, that is not guaranteed, the plane could be 1+ hours late, immigration could be even longer. Read this newspaper report: https://www.thelocal.fr/20170712/airlines-sound-alarm-over-chaotic-border-queues-at-paris-airports

If you are looking to book an advance nonrefundable ticket, Don't try for the 8:59 connection. If you want to take a chance (and if the discount is truly significant), book the 11:20. Otherwise, just buy the ticket when you arrive at the CDG station. If the stars are aligned properly, you could make that 8:59 train.

You have 3 options:

1 Buy a cheaper non-refundable/changeable ticket for a departure time that gives a seemingly reasonable time for landing & clearing C&I - taking a chance you might miss it and lose out

2 Buy a flexible ticket now which can be changed for free up to 2 hours after departure if you need to change the time

3 Wait & purchase when you arrive

Thanks, Chris, for the article. Most enlightening, to be sure. Thanks to everyone for the suggestions to buy the tickets there. Being creatures of habit, we usually buy in advance for peace of mind, but it sounds like that might not be the case here.

It is much cheaper to buy early. Much. but I would not do that unless I was allowing a minimum of 3 hours from landing to departure of the train. And that is no guarantee but at least odds are fair. Two hours is not enough time; immigration can easily take two hours on a bad day and an hour on a typical one. Planes are often half an hour to an hour late. (of our last 10 European trips, 8 were late) I would probably buy early if I could get 3 hours after ETA and could get prices about a third of last minute tickets and take the risk. But be prepared to have to buy a second ticket if you do that.

If you are catching the train at CDG, I'd expect about an hour minimum to get to the train. It takes me about that to get to the RER and I have no checked baggage to claim. With checked bags and allowing time for potential unforeseen delays, I wouldn't schedule less than two hours. It wouldn't hurt to give yourself a little more time than that. Why be stressed about potentially missing your train?

I've been checking various sites for the answer to this with no luck: if I book the 11:20 train with the added surcharge for flexibility, but get to the station prior to the 9a.m. departure, am I able to get on the earlier train?

Brad estimates that he can make the train in an hour not only because he doesn't check baggage, but also because he knows CDG and where the train is relative to Delta arrivals. If I were arriving there for the first time, I'd allow myself more time. (The way to the trains is clearly marked, but it just takes more time the first time before you really know where you're going -- even inasmuch as getting oriented when you're in the train station itself, much less getting to it.)

I would give myself 3 hours to be safe. If your flight arrives on time, you'll probably make it to the train station well before then and have a bit of a wait. But if your flight is late, you'll be grateful for the cushion.