For AAdvantage flyers, the usual format is to fly into FRA and then take the dB to your destination in Germany.

Have a paper copy of your itinerary with all the details, as dB sometimes wants to clip or mark something.

Hold onto the papers/tickets with the clip or mark, as the dB "miles" are sometimes problematic to post and require a note to the AAdvantage desk.

The transfer from the aircraft to the train in FRA is pretty easy, although the first time you do it you may need to ask directions to get down onto the tracks.

Personally, since LH stopped flying the 146s to my destinations out of FRA, I have preferred the dB option - it is relatively quick, you can get up and move around, and some of the vistas in the old East Germany are really very lovely. Especially the ripening fields of rapeseed and the hedgerows - I imagine how enjoyable it would be to walk the hedges with a shotgun and a dog (although I don't know whether Germany actually has any pheasant or grouse or even a hunting season).

Also, some of the railroad stations in Chemnitz, Dresden and I think Leipzig have cafes in or near them that will serve good coffee and German sandwich-y sort of things. Far better than their respective airports, anyway, and their breakfast offerings for an early morning departure are more flexible in timing than some of the hotels I use - hotel breakfasts don't start until 6, say, and I need to leave the hotel by 6:15, etc. etc.