Frankfurt to Koblenz train

You said "we". Does that mean two adults?

I would not take an ICE (or any other long distance train - ICE/IC/EC). An ICE will only save you about 20 minutes and costs a lot more. Advance purchase tickets for long distance trains (Sparpreis or Super Sparpreis are less expensive but are train specific. If your flight is late arriving and you miss the specific train (departure time) that the ticket is for, you lose the use of the ticket and have to buy another one at full price.

What I would do (and have done) is to purchase two RMV (Frankfurt regional transit district) tickets for regional trains from FRA to Mainz Hbf for 6,90€ each and a Rheinland-Pfalz-Ticket (an all day pass for regional trains in the states (Länder) of Rheinland-Pfalz and Saarland for 40€ for 2 people (30€ for person 1, 10€ more for each additional adult, up to 5 total). That's 26,90€/person.

The direct Regional Express (RE2) to Koblenz leaves every two hours at 24 minutes past the even hour. If you miss that train and don't want to wait for the next RE, you can use the RMV ticket on the next S-Bahn to Mainz Hbf and catch the next regional train from Mainz to Koblenz with the R-P-Ticket.

Yes, my husband and myself.

If you go to Bahn.com and put in your date of arrival you can see what trains from Frankfurt Flughafen (airport) to Kobleenz Hauptbahnhof (Main Station) are available, and their prices. It looks like you maybe save 15 to 40 minutes by taking the fast train. If you just go pick up a regional train after you are through immigration you won't lose much time, and I expect you can pick up the tickets at any kiosk, no need to book ahead. Have fun.

For the fast train, yes, buying tickets results in a cheaper fare, however it is risky, will your plane be on time? will immigration take longer? will you get a bit lost getting out of the airport and to the airport train station?

For the regional train, no real advantage buying ahead, so less a worry.

The fast train really does not save you much time, the regional train is more scenic, going down the Rhine River valley (The fast train uses a separate high speed corridor)

You might consider getting a Deutschland ticket at ~60 euro each. You will spend at least half that on a regional train ticket to get to Koblenz, and it will be good for any regional train, S Bahn, U Bahn, and regional or city buses and trams. Good for when you set off on your own, or for post tour activity.

It looks like IC 2242 runs along the river on the opposite side of the regional train. If that train fits your schedule, I would choose that one. We took the regional from Boppard to Frankfurt and it had a lot of stops. The day we were on it there were no seats for the first 90 minutes. It wasn't the pleasant train ride down the valley that I had envisioned.

You should follow Lee's advice above for ticketing.

The RMV ticket at € 6.90 each covers you on ANY regional train (S, RB, RE) as far as Mainz. Buy it at any ticket machine at the FRA REGIONALBAHNHOF station or using the DB or RMV app. The RMV ticket will expire after a set period of time, so use it right away on the next REGIONAL train. The Ticket machines will look something like this:

https://www.rmv.de/c/en/shop/more-purchase-options/rmv-ticket-machines

The RE2 is the regional express train and it runs all the way from the airport to Koblenz Hbf (main station in Koblenz. It runs at 7:10, 9:24 and 11:24 on most days. No changes to make. No seat reservations to make either (none are possible.) I would buy my train ticket about 30 minutes before the train departs and wander down to the correct platform after that. Don't delay boarding - this ticket expires after a set period of time.

CAREFUL NOW... The 2nd ticket Lee suggests is the Rheinland-Pfalz ticket, a day pass and good all day long, and it is also good on any regional trains that day so it covers you the rest of the way between Mainz and Koblenz. Buy this ticket ALSO before boarding the train at FRA, either from a ticket machine or using the DB App. You do not say what time of day your flight is schedule to arrive. Is this a WEEKDAY? If so, the R-P ticket is not valid until 9 am, which means that the 7:10 departure is off limits for the RE2 train, as it will pass through Mainz will before 8 am. You will need to wait at the airport a while if you wish to ride the 9:24 RE train or the 11:24 train... On Sat or Sun, there is no after-9-am rule, so ride that 7:10 train if you wish.

Do check the train schedules yourself for the correct date, time, etc. I used only one date (May 10) for the examples I used.

Where is your hotel? The Rheinland-Pfalz ticket is also valid for any BUSES you might need to reach your hotel in Koblenz that morning.

About this comment above...

"The fast train really does not save you much time,"

To Koblenz, there are no direct "fast" trains (which I assume means ICE or EC or IC trains, better known as long-distance trains.) There are some journeys which include long-distance trains, but they also include regional trains and a transfer on the way. The Regional Express train is the absolute winner when it comes to speed!

"It looks like IC 2242 runs along the river on the opposite side of the regional train. If that train fits your schedule, I would choose that one."

There are in fact regional trains on both sides of the river. I would not choose any IC train with a saver fare ticket for the same reason others have given - you have no time flexibility with these tickets, which will be forfeited if you miss your train due to a flight delay or security/baggage issues. And besides, there is no IC 2242 from the airport to Koblenz, AFAIK. I see ONE direct IC train, the 2241 at 06:58. It makes one stop in Bingen on the way and runs on the same side of the Rhine as all the other long-distance trains do. I guess that might work if you were scheduled to arrive at FRA by 6 am, and if you actually arrived at that time, at a terminal not too far from the railway stations, and if you had no problems at security or baggage. But that is a lot of if's.

I’m 0 for 2 in trying to ride an ICE train after flying from USA to Frankfurt. Take the RE and get your ticket after landing