We will be traveling to Frankfurt late July, arriving on a Tuesday morning from US at 7:20a. We need to be in Berlin by night, but our arrival time in Berlin is open. Assuming our flight is on time, I was looking at taking the regional train that leaves at 9:30a to Mainz, then RB26 up the river to Koblenz. Looks like we would arrive in Koblenz around 11:30a, visit in Koblenz for a couple of hours for lunch/sites. Then regional train RE5 to Cologne (looks like one leaves around 2:15p), and then ICE train to Berlin that leaves around 6:45p and arrives in Berlin around 11:00p. Is this doable and would we have sufficient time to store luggage at train stations, eat lunch and visit a few sites in Koblenz with the 2hr 15 min window and see the basics of the Cathedral in Cologne in the 2hr window in Cologne. Over ambitious?
That is very much a trip with little time to actually visit the Rhine Valley. Also, doing this on your day of arrival, if you flew in from the USA, you will likely have jet lag and be tired.
Why not take a day or two for your trip to see more. Most of what you will see will be from the window of the train.
You will have to store your luggage and that will take time. If you plan a 2 hr window in Cologne, you won't have much time there, since you will have to arrive back at the train station half an hour before your next train.
"We will be traveling to Frankfurt late July, arriving on a Tuesday morning from US at 7:20a. We need to be in Berlin by night."
I don't see the travel gymnastics or 8 hours (that's 8 just on the trains!) or the stops you have in mind here as particularly rewarding. Nor would I want to undertake them in the sleep-deprived and possibly jet-lagged state that comes after a red-eye flight.
I would stick to a more direct / standard Frankfurt > Berlin routing... A flex ticket for a train that makes stops on the way to Berlin (about 4.25 hrs. train time) would allow you to step off at will (if you feel up to it) in an interesting town like Goettingen or Hildesheim without adding train travel hours to your day.