Train takes about 4 hours. Others will disagree, but having lived nearby briefly, I personally don't find Frankfurt particularly interesting (especially for any longer than a day), so I'd continue straight to Berlin or stop somewhere like Hannover along the way instead.

There´s no direct train from Frankfurt airport to Berlin.
Either you go to Frankfurt main station (from there, it´s a direct train to Berlin every hour) or you have to change the train (mostly in Hannover or Leipzig, but it depends which connection you have) if you leave from the airport.

Frankfurt is a forgettable city. But it is a pretty good base for excursions due to its central location. Whether it's Heidelberg, Rothenburg or Loreley..many tourist favourites are comfortably within day trip reach from FFM.

What if I flew from Atlanta to Amsterdam or Paris. Would the train trip be more interesting to Berlin?

Well strangely enough there is a direct train Paris to Berlin, leaving Paris Est early morning and arriving Berlin late evening. You would need to get from CDG into town. It's a 12 hour + trip

And there are trains directly from AMS every two hours changing in Hilversum with direct connection to Berlin. This takes about seven hours

As stated above neither of these is a particularly quick or scenic route - all you would see is a lot of rather flat countryside.

There hasn't been a through daytime train from Paris to Berlin since 1981 (the old Leningrad Express), and the overnight train ceased operation in December, 2014. There are enough fast connections in Cologne or Mannheim however.

The Paris-Moscow train leaves Paris Est at 08.28 and arrives in Berlin at 21.29 on platform 1.

There have been conflicting reports on whether this train takes passengers between Paris and Berlin. In the past it did and then it didn't. The DB website (bahn.de) currently does not show this connection and will not sell tickets for it, nor does the SNCF website. The European Rail Timetable (ex-Cook's) shows the stop in Berlin, as does the DB-Fahrtinformationen for EN453, but my understanding is that is only for passengers traveling from Berlin to Moscow, for which bahn.de will sell a ticket. I hear that in the past people have managed to ride the train from Paris to Berlin by buying a ticket to Hannover or Frankfurt (Oder), but these tickets are no longer sold. SNCF won't even sell tickets on the train to Warsaw, but I suppose one could buy a ticket from Paris to Brest for 172 Euros, which is cheaper than the full fare Paris-Berlin, and just jump off the train in Berlin. There is still no daytime, direct service from Paris to Berlin.

Probably a good and accurate observation. I used the facility Paris to Frankfurt (south station) with my LH boarding card as a ticket during a strike which was either French ATC or LH pilots or cabin staff (can't remember) about eighteen months ago. No problem with it being accepted at the time, but perhaps a degree of mitigating circumstances. I sat the whole way in the restaurant car (IIRC the rest of the train was sleepers only) with a very pleasant fellow who went on to Berlin with the same kind of "replacement travel documentation" that I had. Maybe I this was a situation where there were deviations to the norm.

That said as you accurately point out you can get to Berlin quicker by changing in Mannheim or Cologne if using rail travel, although flying is likely to be quicker and cheaper