I'm late to this thread, as you've already purchased your tickets. But I have some ideas for you to consider for the future.
One is to take a nonstop flight followed by a train ride. For some routes, you can even book this as one ticket, so you are "protected" (they will put you on a later train at no extra charge if your incoming flight is delayed). I know that many airlines into Germany will allow you to add a train ticket to Germany and some neighboring countries in this way, in a program called DB Rail and Fly. Alas, all airlines are not participating, and it seems Delta is not (Lufthansa from Atlanta would work, though). See here for some details: https://www.bahn.de/p/view/service/buchung/flug/rail-and-fly-english.shtml
Similarly, you can buy flights on Air France that include a flight to Paris followed by a train ride to other French cities, all on one ticket. Details here: see the tabs for Strasbourg, for Brussels, and "other train links": https://www.airfrance.us/US/en/common/resainfovol/avion_train/reservation_avion_train_tgvair_airfrance.htm
If you can't get a fly and rail combo ticket, you'll probably want to wait until arrival to purchase the train ticket, so you're looking at last minute prices and availability. In turn, for some routes that's expensive and there may be sellouts, and for others it's not a problem.
For instance, looking at last minute tickets from Rome Fiumicino airport station to Milano Centrale station, it's €106, and about 4 hours (exact times varies) with one train change. If you had been able to get a decent nonstop fare from Atlanta to Rome, and can live with the added cost of the train, that's not a bad plan.
A few years ago, I had miles to get to Rome, but not to Sicily, so I had to use separate tickets for that leg. Looking at the Rome to Sicily flights, the prices were climbing fast (I was arriving on a Monday morning). What I did was buy two tickets, one for three hours after my scheduled arrival, and one for ten hours after. I knew I'd use one and throw one away, and the combined cost of the two tickets was still cheaper than a last minute ticket would have been. On the way home, I had to spend a night in Rome to make sure I didn't miss my flight back to the US (much more expensive at the last minute than I wanted to spend!).