Frankfurt Airport to Koblenze central train station

It's extremely straightforward. Just follow the signs to the Squaire. Train station is downstairs.

It is slightly more complicated than that. Right now, there is a regional direct train every 2 hours (RE 2), and it goes from the Regional Bahnhof. It goes at 24 past the odd hours (7:24, 9:24, etc).

There is also an ICE train every 2 hours, and it goes from the Fernbahnhof (long distance station), It goes at 59 past the odd hours.

The two stations are adjacent to each other. You should download the DB app, so you can check when the next train is after you get your luggage. That way you will know which station to go to. And do not get hung up on a direct train. The next train when you are ready may have one or two connections.

To be honest, whether you take the Regional train with lots of stops, or the ICE train with one stop, there is not much time difference, you maybe save 20 minutes with the ICE train (1 hour 10 vs 1 hour 30), but I think the regional train is a bit more scenic. There is also a third option that requires a change, but likely not worth it.

As for getting to the train after landing, it depends which terminal you come into, follow the signs, you will either just walk there, or need to take a free shuttle bus.

Look on https://int.bahn.de/en/ for options, I would be hesitant to buy tickets ahead, unless it is a regional ticket, but I would wait until I get there and pick the soonest best option.

Note that the regional and the high speed stations are different, but only across the road from each other and connected.

The names of the stations - in case you look them up in www.bahn.com are Frankfurt Flughafen Regionalbahnhof (the regional station) and Frankfurt Flughafen Fernbahnhof (i.e. long-distance train station). Flughafen (airport) will be abbreviated to 'Flugh'. Above the station in The Squaire are a bakery, a café and a Rossmann (comprehensive drugstore without pharmaceuticals). There is also a Hilton and a Hilton Garden Inn, on another floor. You can wheel your bag from the airport to the trains the whole way, and there is an escalator going down into the station. There will be signs but I have had to ask people in the airport how to get to The Squaire, which is just adjacent to the airport but connected. There are helpers everywhere, so don't worry, it's not hard once you know where to go.

Lavandula

You should not overthink this. While it is true that FRA has two train stations they just operate as one. When planning trips just enter "FRA" as originally, and your destination. In this case Koblenz. The system will show departures from both stations, and will show the platform numbers. As long as you follow the signs to the right platform nr. you will be fine. Platforms 1-3 are in the older regional part, and 4-7 are in the flashy new high speed part.

This is all pretty straightforward. If you want you can just go to the DB ticket office, and get tickets and instructions there once you have arrived.

yes, getting on the train of our choice is simple. But first there is the matter of choosing. The higher quality ICE trains do not accept tickets for regional trains, if you buy at the station there is a €4 price difference. Make sure you are buying what you need.

Rather madly if you buy in advance the higher quality trains can be cheaper, but those tickets will be locked in to the specific departure and that brings in the age old worry of a delayed flight.

PS I've argued this principle with them before, that time over London Heathrow trains. It seems reasonable to me that when producing the details of their premium priced tours they should explain these transfers properly and not expect customers to start hunting through guidebooks and forums.

Expanding on the post above, I would never take a long distance train, like an ICE, on the day of arrival in Germany. I only take one of these trains when I can get a lower fare with an advance purchase, but those fares are train specific (day and time), so taking one on the day of arrival puts you at a risk of loosing the cost of the ticket if your trans-Atlantic flight arrives late.

While it is true that FRA has two train stations they just operate as one.

Yes, but usually the Fern Bahnhof serves only long distance trains (ICE, IC, EC). You don't want to take one of these trains, so forget the Squaire and the Fern Bahnhof. Use the closer in Regional Bahnhof.

I did something similar to what you want to do once, to Treis-Karden on the Mosel, via Koblenz. Go to the Regional Bahnhof. It's under the building across the street from Terminal 1 at FRA, with access down stairs from the lobby of T1. The actual tracks are a floor below the ticket level. Purchase two tickets, an RMV ticket from the Regbf to Mainz Hbf, for 6,90€/P, and a Rheinland-Pfalz-Ticket, and all-day ticket valid on any regional trains in the state of Rheinland-Pfalz (Mainz and Koblenz are both in Rheinland-Pfalz, as is all the route between them), for 29€ (that's for the first traveler, each additional person, up to 4, is 9€).

Look at the schedule for trains leaving the Regionalbf. In my case, I took the first S-Bahn to Mainz, from which I caught an RE to Koblenz. But you might find there is an RE direct from the Regbf to Koblenz, or you might get a regional train to Ingelheim or Bingen and another regional train from there to Koblenz. Regardless of whether you actually change in Mainz, you'll go through Mainz and you'll use the RMV ticket to Mainz and the Rheinland-Pfalz-Ticket the rest of the way.

If you do change trains in Mainz, just a word of warning. A lot of trains going to the Rhein gorge leave from tracks 11-13. These are not through tracks out beyond track 8, but stub tracks that come in from the NW and end against the station building, to the right (in the direction of travel) of your arrival track (probably 1-4).

A 2nd class RE ticket is 37.80 EUR for one person, always the same price. The Flex ticket (walk up fare) for the ICE train is 44.30 EUR, That is a 6.50 EUR price difference, nothing to get too excited about. So if the ICE train is the next available train, just take it unless you want to wait around another 1 1/2 hours for the next RE2.

A 2nd class RE ticket is 37.80 EUR for one person. The Flex ticket
(walk up fare) for the ICE train is 44.30 EUR, That is a 6.50 EUR
price difference, nothing to get too excited about.

First, as I just explained, an RMV ticket to Mainz plus a Rheinland-Pfalz-Ticket is 29€ + 6,90€ = 35,90€, not 37,80€. Plus, you don't have to wait around for the next direct RE to Koblenz; there are three more connections in the next 2 hours that will get you there sooner, for 35,90€.

And, if there are two of you, the full fare differences get worse. A regional fare is 2 x 37,80€ or 75,60€. A full fare ICE/IC ticket is 2 x 44,30€ or 88,60€. Using the Rheinland-Pfalz-Ticket, as I showed, it's 38 + 2 x 6,90 = 51,80€, for two.

So if the ICE train is the next available train, just take it

But, if you are in the Regionalbahnhof, it's a 6 minute walk to the Fernbahnhof.

Yes, I'd just take the next regional trains to Koblenz.

Depending on how much luggage you have, and what your appetite is for changing trains, you may want to focus on direct trains. There are direct Regional and direct ICE trains. Direct regional takes 1h 59m, Direct ICE takes 1h 39m - not a lot of difference. You have the option to reserve a seat on the ICE, but not on the Regional. I believe the reservation will cost you about 5 Euros. We opted for ICE, and bought a seat reservation just to be sure we got a seat.

Google Maps is very good at showing German train options - be sure to set the 'depart at' to an appropriate time when planning from a different timezone, otherwise it's going to be showing you what the situation is at 3am! Sample Google Maps for the journey. Be aware they will show lots of options; not just journeys with multiple transfers, but also Flix bus, etc - focus on the journeys that are direct (no changes).

Also note that the official 'DB Navigator' App for iPhone and Android is fantastic! Really good at showing you useful information, and lets you buy tickets. Possibly the best transport-focused App I've ever used! Make sure you get the official DB Navigator app from Deutsche Bahn; when you search the store, lots of wannabee apps will show up, some sponsored to show higher than the DB Navigator.

Be aware that trains in Germany are no longer legendary in terms of on-time performance. Possibly due to their offering amazing regional passes and deals, their ridership has swollen and their ability to carry that many passengers has suffered. Typical discussion about it here - https://community.ricksteves.com/travel-forum/germany/german-trains-are-they-as-bad-as-i-m-hearing