Time needed from international flight and train at CDG on a Tuesday morning

Like Clint Eastwood said, "Do you feel lucky?"
You say "Landing at CDG at 7:15 from US". You don't know that, only that the airline is planning to land at 7:15.
Where are you going? You could just see if you make it and buy a ticket once you are at the station with your luggage.

Angers. Have only ever purchased train tickets ahead of time online. Surely it's not too difficult to purchase there. Thanks good thoughts

You should always add 30 minutes just to deplane.

Do you need to pick up luggage?

Then there is customs.

And CDG is a very large airport with multiple terminals.

This website says minimum transfer time is 90 minutes.

It is not difficult to purchase train tickets at CDG. If you are traveling in economy, odds are you will not make the train in 90 minutes but you never know. Anything could happen especially if your flight lands early and a gate is available for your plane (recently, I landed 45 minutes early and we then spent 45 minutes on the tarmac waiting for a gate). No way will you make it if you have checked luggage unless you are super lucky and I do mean super lucky.

It's not difficult to buy train tickets in France, but you do have to queue up to buy them. it can take time. So if that is your plan I think the 8:45 becomes much less likely.

Day-of tickets are also generally more expensive than advance tickets, if that is a consideration. If it isn't, you might consider refundable tickets, purchased in advance. Or buy a cheaper nonrefundable ticket and have a plan B if you miss that train.

It's not difficult to buy train tickets in France, but you do have to queue up to buy them. it can take time.

Or buy them with your smart phone using the SNCF app.

I wouldn't try it. The last two times I flew into CDG (both were this summer) there were quite long lines for passport control - no less than 30+ minutes to get through. Luggage has also been painfully slow to get to the carousel - more than 15 minutes waiting for the first piece to come out. If your plane is late that could also be a problem...it's been known to happen! Some airlines also bus you from the tarmac to the terminal...that can add an additional 15 - 20 minutes.
Use the app to buy your train tickets, either from SNCF or trainline.com. That way you have a QR code and don't have to worry about purchasing paper tickets when you get to the airport - plus they will be cheaper.
Either way I would book a TGV no sooner than 2 hours after your arrival time. That would be my personal comfort zone at least.

The plane taxi routes around the cluster of terminals can be time-consuming. If you are near the rear of the plane it will take time to get off. No way of knowing about border entry procedures or time to collect your bags. It can be a long walk to the train station in terminal two if you arrive at another terminal. So it is "reasonable" to believe your chances are risky. You can buy a flexible train ticket on some routes but at a substantial extra cost, and you will still have to arrange a seat reservation on the next train.

FYI, the scheduled arrival time of a flight is the arrival time at the gate, not the landing time. If the scheduled arrival time is 7:15, then they'll estimate average time from landing to gate and built that into the schedule. If the average is 30 minutes, then it should land about 6:45.

If I check bags, I estimate one hour from the landing time of my transatlantic flight to the time I'm through passport control and have picked up my bag(s) and am on my way to seeking my transport onward (bus or RER or taxi into town). 8:48 would really be pushing it and 1) that depends on the plane's landing on time and 2) I know my way from the various terminals to the train station at CDG.

Landing at CDG at 7:15 from US and hoping to make an 8:48 tvg
connection. Is that reasonable?

Nope. And Sam's quote of a classic Clint Eastwood line sums it up brilliantly.

It would be extraordinary good luck to make that train. Your plane would have to be early; you would have to have a quick passage through immigration (we average about an hour, but it can be 10 minutes if you are lucky and two hours if you aren't) and you would have to have no checked baggage as that takes a while. Only try it if you are comfortable forfeiting the train ticket and buying a new expensive last minute ticket. The rule of thumb at CDG is 3 hours between landing and a train from CDG.

Buying a ticket involves lines, often slow long ones and last minute tickets on the TVG are much more expensive than those bought several months in advance; the advantage of buying on the spot is that you don't risk losing the ticket if you haven't left enough time to make the train.

I wouldnt bet on it. Maybe a 10am would be more optimistic. 11-12 to be safe.