Trains

You can and should buy a roundtrip train ticket from Frankfurt airport (Flughafen) to Cologne (Koln) on the day you arrive. There are direct trains.
You can search the schedule and buy tickets on bahn.com. There is an ap you can use to buy tickets in advance once you are sure that you will be able to catch the train at the hour you want. You may need to use the German spelling for the city or stop.

Check wether your airline is participating in the Rail&Fly program of the Bahn. So you could get a flexible train ticket for about half of the standard fare.

cdbusseau, you're right, a car is not necessary. Train travel is easy, and doesn't have to be micro-planned. Go to the German rail website, bahn.com, and you can input departure stations and destinations, dates & approximate times, and it will come up with all the available trains and connections. There are many trains a day that go up and down that corridor. You can buy tickets in advance, but also right there at the train station in the Frankfurt airport when you arrive. Its like the subway in New York, or a city bus system, where you might have to transfer, but its pretty easy to figure out. Look at the Travel Tips tab on this page, and the seat61.com for more tips.

But if you're asking if Cologne (Köln) is a good place to go for four nights, I think many people (including me) would say there are more interesting places to stay near Frankfurt. Cologne has a famous cathedral but otherwise its just another big city with a nice riverfront. Many people (including me) would recommend the middle Rhine area (Bacharach, St Goar, Koblenz, Rudesheim, etc.) as these are nice small towns on the most scenic part of the river, with cruise options, vineyards, castles, etc. A good guidebook will help describe the towns, as well as the train travel part of this.

Stan's got it right - train to the Upper Middle Rhine Valley between Koblenz and Bingen.

Sadly there's no such thing as a good guidebook for using the trains there. But it's not rocket science. See map of area with train routes on each side of the Rhine:

http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php

Boppard is an attractive old world town with lots of hotel and dining options as well as a good base town - and it offers FREE transportation locally for most outings.

You can get a ticket for the train ride to Boppard from the airport station. Head to the Regionalbahnhof (one of two airport trains stations) and use the ticket machines or buy at the counter. It's cheapest if you get a RMV ticket to Mainz and a Rheinland-pfalz ticket for Mainz > Boppard (repost if you want the details.)

All Boppard innkeepers will hand you a Guest ticket good for free transport during your stay. St. Goar (Rheinfels Castle), Oberwesel, Cochem, Braubach (Marksburg Castle) and Bacharach are all within very easy reach by train; of these, only Bacharach would require a supplemental ticket to the Guest Ticket.

Info on the Rhine towns: http://www.loreley-info.com/eng/rhein-rhine/city-cities.php

If you want a day trip to Cologne for the cathedral and museums, you can do that from a Boppard base. Or you could go there on your way to Frankfurt at the end - stow bags in a station locker. You would need train tickets for Boppard > Cologne > Boppard or Boppard > Cologne > FRA airport. Buy in advance for savings at the DB website:

https://reiseauskunft.bahn.de/bin/query.exe/en

Let me throw another idea into your head.....Berlin is a 1:10 flight from Frankfurt airport. And since you are already at the airport.....

Fabulous information, everyone! RUSS & STAN- we want to know more. I’ve been looking at maps and town descriptions- Bacharach, Boppard.....much to choose from. Any more pointers? We are thinking somewhere along that route and we can take the train to other spots, maybe a day trip to Cologne? Thank you so much for the links and information. We will be tired upon arrival to Frankfurt, so the more info we have to help us find what we need, the better.

Any more pointers?

Well, I didn't say much about the Mosel (Cochem)… It's a beautiful town that can be combined with a visit to Burg Eltz Castle. The free Guest ticket will get you there (one change of train in Koblenz.) There's a trail from Moselkern train station (town of Moselkern) to Burg Eltz castle that takes about an hour. Nice walk. Or you can catch a shuttle bus on weekends from another town called Hatzenport (also has a train station.)

Bundesbank Bunker in Cochem

Reichsburg Castle in Cochem

Rhine/Mosel train line map so you can find these places:
https://www.vrminfo.de/fileadmin/data/pdf/2018/VRM-Schienennetzplan_2018.pdf

Come back with questions after you have a look into some of these places.

cdbusseau, yes its easy to do day trips by train such as to Köln, just note how long the roundtrip is so you can pace yourself. Have you ever seen those commercials for Viking River Cruises, where you see the ship going through the narrow river valley with mountains on both sides, castles and vineyards, etc.? Well that is the Rhine (Rhein) Valley in the area of those places mentioned. Bacharach is a small, quiet town (a leisurely 15 minute walk from one end to the other) on the west bank. There is a non-tourable castle above the town you can hike to for the view. Wine and more wine. Hop on the train or ferry and go downriver to St Goar for another castle, the famed Lorelei rocks and just a fun cruise. Or stay in St Goar and do the reverse. Oberwesel, St Goarshausen and other towns along there are linked by trains and boats. Rudesheim, and Boppard are somewhat bigger towns with more options. Rick Steves is a big fan of the area and writes about out extensively in his Germany guidebook, if you need more description and hotel/restaurant recommendation. But its not undiscovered - very popular - so I'd be booking a hotel soon.

Yes, you don't want to do a lot after just arriving at the airport in Frankfurt, but it will be morning there, so you should just keep moving and take a train right from the airport (Flughafen) rail station (Bahnhof) and get somewhere you want to be. There are English-speaking clerks at the train station in the airport if you need help getting oriented. Just note the train station in Bacharach is unmanned so any travel you do from there, you need to buy tickets from a machine.

Bacharach is a small, quiet town (a leisurely 15 minute walk from one
end to the other) on the west bank. There is a non-tourable castle
above the town you can hike to for the view. Wine and more wine. Hop
on the train or ferry and go downriver to St Goar for another castle,
the famed Lorelei rocks and just a fun cruise. Or stay in St Goar and
do the reverse.

Stan's comments offer Bacharach as a base. That in fact is the one Rhine town that RS makes quite a large fuss about, and consequently, where a lot of RS's readers end up staying. Stan and Rick also share this idea of hopping on a boat in Bacharach for a cruise to St. Goar - and I'm sure many people do just that.

But this strategy terminates your cruise a mere 45 minutes after boarding - and while it's very scenic, it skips half of the most scenic part (the part south of Bacharach and north of Bingen.) IMHO it's much wiser to spend the extra 45 minutes needed on that part as well by doing a Bingen > St. Goar cruise. See the cruise map of the most scenic part.

So if you make Bacharach your home base, your best cruise is had by 1) taking the train south to Bingen, 2) cruising north to St. Goar, and then 3) catching a second train back to Bacharach.

My take is that Boppard (or St. Goar) is a preferable base town for other reasons as well, but also on the grounds of a simpler cruise outing. One train south to Bingen, one cruise back to St. Goar; now you have seen the WHOLE best part, and you're "home" if St.Goar is your home base... if that's Boppard, then one more train to Boppard (for free with the Guest Ticket.)

If you stay in St. Goar, it's possible to jump on the cruise ship there and head south to Bingen, but this "in reverse" route is a problem. One train south to Bingen, one cruise back to St. Goar is more time efficient because the cruise itself, if southbound, requires nearly 3 hours instead of 1.5 hours northbound. It's a very strong current.

Bacharach's old-world buildings are the nicest around - you should definitely pay the town a visit if you have time, I think. If you stay in St. Goar, or in Boppard, one option would be to break up the train trip to Bingen (for boarding your cruise) with a few hours in Bacharach. Or you could just take the 10-minute train ride from St. Goar (20 from Boppard) to Bacharach at some other time.

About ferries: The map I provided earlier at http://www.loreley-info.com/eng/rhein-rhine/walking-hiking.php shows ferry boats here and there in RED. Those are ferry CROSSINGS. Most take both cars and passengers, but only ACROSS the river. When Stan says "Hop on a train or ferry..." he is actually referring to the day cruise ships, operated by KD and others, which travel upstream and downstream.

My wife and I had a great time staying in St. Goar at a small hotel right down by the river, in March so it was the off season. We also popped into Boppard and there was an interesting hotel down by the river which we may go to this year. The ferries were great and cheap. You could catch one up or down river, then return by train each day. We are also considering the off shoot Moselle branch, checking out Bielsen and Cochem,before training down towards Cinque Terre through Switzerland..anyone have suggestions for that leg let us know..happy travels..John