I'm contemplating a trip that includes Frankfurt and Berlin as my gateway cities for air travel, and going by train in between. And, I'd like to visit Christmas markets along the way if possible, but I'm not clear on what's the normal "route" of the trains between Frankfurt and Berlin. What about Berlin-Dresden-Frankfurt? (trying to think creatively). Thanks!
The ICE pretty much goes straight to Berlin from Frankfurt in 4 hours. It does not stop at Dresden. Now, if you are taking the slower Regional trains using one of the cheap tickets like Quer Durch Deutschland or Happy Weekend, the trip would take you 8 hours or so and then it might make sense to get off the train somewhere.
If you use both Frankfurt and Berlin as bases, you can visit many of the smaller towns around them using the S-bahns or the cheap Regional bahn tickets, if you want to see more Christmas Markets. For Berlin, think Luebeck, Quedlinburg and Potsdam. For Frankfurt, think Mainz, Wiesbaden, Bad Homburg, Hanau, Aschaffenburg, Selignestadt, Heidelberg, Wuerzburg, Koenigstein, etc.
The normal route for an ICE from Frankfurt to Berlin is via Hanau, Kassel, Göttingen, Braunschweig, Magdeburg and then straight to Berlin. Frankfurt has got a great and very traditional Christmas market and it's one of the oldest anyway.
Schedules for trains in Germany can be found on the German Rail website. To get Frankfurt-Dresden-Berlin use the via "Dresden" and the time you desire to stay in Dresden. You can go by ICE to Dresden and EC to Berlin in 8 hrs with 45 min in Dresden. Those are probably hourly trains, so 1h45m in Dresden would be a 9 hr trip.
You can use a single Dauer-Spezial ticket for the whole trip, but you have to schedule the stop in Dresden using the "Via". Then it will be a single ticket for both trains for as low as €29. Note that a Dauer-Spezial ticket is train specific; you cannot stay longer (or less) than planned in Dresden.
The fastest train from Frankfurt to Berlin is the ICE sprinter which makes the trip non-stop in about 3½ hrs. There are a lot of connections at 4 hrs, either direct via Braunschweig or with a connection in Hannover.