Train Question

You can't "hop on and hop off" long-distance trains that require reservations -- but you CAN book a set of trains with a longer connection (did this successfully to stop in Wuerzburg on the way to Frankfurt for about 4 hours.) It's in the "other options" section, as I recall. But you MUST take the specific trains you booked, otherwise the good price doesn't work.

You should buy a through ticket and request an intermediate stop of a defined duration under options. However, that's necessary only if you travel on a saver fare ticket (bound to a specific train). If you have a flexibe ticket, you can break your journey at will. And even if you travel on a saver fare ticket which includes an ICE for the Frankfurt - Köln leg and an RE (regional train) on the Köln - Aachen leg you are free to take any regional train from Köln to Aachen (but not a Thalys or ICE).

@Laura B: none of the ICE's between Frankfurt and Köln requires seat reservation.

Laura B's comments refer to the Sparpreis or saver fare tickets available online in advance at the DB website. To get these discounted tickets, you would book Frankfurt Airport to Aachen. But to do as you intend, you would also book a stopover in Cologne for a certain duration (say 2-3 hours) into your schedule. If you don't book the stopover, then the booking system provides the journeys with the best connections for your trip (thus, the shortest stopovers) and you are bound to the train schedule (assuming both the FRA-Cologne train and Cologne-Aachen train are long-distance ICE. ICE, or EC trains.)

If you pay full price for your tickets - the flex price, which you can buy at the FRA station if you like - then you can stop over at will on the same day as long as you use the same route.

Something important to consider: flight delays, cancellations and such... if you are flying into FRA that same day, the Sparpreis fare may be lost if you can't make your first train out of FRA. Then you have to buy a full-fare ticket anyway. I know you want to get to Cologne and Aachen, but it may be wise to stay somewhere closer to FRA that first day. Mainz is a good idea. You can get there in less than 25 minutes for less than €5. Relax, see the handsome old town, have a nice meal and a good sleep.
http://www.master.economics.uni-mainz.de/augustinerstrasse-abends.jpg
https://c1.staticflickr.com/9/8517/8466997951_748e0b61f0_b.jpg
http://www.neumainzer.de/wp-content/uploads/2015/06/Besuch-auf-dem-Mainzer-Wochenmarkt.jpg

Then take the train the next morning through the scenic Middle Rhine Valley to Cologne and Aachen. This way your Sparpreis ticket would not be compromised:
http://whc.unesco.org/en/list/1066

Of course, if you have a rail pass of some kind in your hand, you can make the journey with stopovers as you please at any hour.

@sla019, thanks for the clarification.

The German trains don't require reservations (seat assignments) but the advance-discount prices do lock you into particular train departures. Rather than counting on your flight to be on time, you can just buy tickets upon arrival. You can book the connection with a longer "stopover" at intermediate stops. See the link to add stopovers on the main DB search page. Savings versus buying each leg separately will vary. If you have other, longish train rides in the plan, then you could also have hop-on flexibility with a German Twin Pass.

As I look at the schedules, all of the connections that have a change of trains in Köln continue on from Köln with a regional train (RE or RB). According to the Vor- und Nachlauf provision of the Bahn, only the long distance trains on a Savings Fare ticket are "train specific". You can take any regional train on the same route to finish your trip to Aachen, as long as you finish it before 10 AM the following day. So you could spend as much time as you wanted in Köln before going on to Aachen.

I also see a number of direct ICE connections between FRA and Aachen with a stop, but no change of trains, in Köln. If you booked one of these connections, you would have to use the specified train straight through.

But you still have the question of using a train-specific ticket on the day of arrival, when the flight could arrive too late to make your specified long distance train. There are three alternatives to purchasing full fare tickets at the airport:

• Stop for the night somewhere like Mainz. Then, the next day use a purchased-in-advance Savings Fare ticket to go the rest of the way. Note, Savings Fare tickets must include at least one leg by a long-distance train (ICE/IC/EC).

• Go the entire distance by regional trains. This avoids the train-specific problem of the Saving Fare ticket, but the trip is longer, around 4½ hours vs sometimes less than 2 hours by fast trains. However, the regional route would take you down the scenic Middle Rhine Gorge (Mainz to Koblenz). Depending on the day of the week, you could use either a Schönes-Wochenende-Ticket (44€ for two on a weekend) or a Quer-durchs-Land-Ticket (52€ for two on a week day).

• Gamble. It's not true that if you miss the specified train due to a late flight you lose all of the money you paid for the ticket. You can still apply most of what you paid for the Savings Fare ticket towards a full fare ticket going to the same place on the same route. At a Bahn ticket counter you can pay a 19€ "fine" and apply what you paid for the Savings Fare ticket towards a voucher for a full fare ticket on a later train. This isn't an open exchange; the voucher is only valid for a train going to the same place by the same route that day. So, for instance, instead of just booking the 11:43 ICE/RE connection for 166€, you could advance book it for 48€. It you make the 11:43, you save 116€; if you miss it you apply 29€ toward another 166€ full fare connection (137€ more).

Versus just paying 166€ for the full fare ticket, ;you'll save 116€ vs risking a loss of 19€.

Here is another modest proposal. As Lee pointed out, there is a direct ICE train that does it in 1 1/2 hours and costs 166 EUR pp for a Flexpreis ticket. Other connections using ICE trains have a change in Cologne and take 2 to 2 1/2 hours and cost the same. The ICE trains run much of the way in the median of the autobahn between Frankfurt and Bonn and get up to 200 mph. You'll be passing Porsches like they're standing still. But the scenery is nothing to write home about.

If time is not really important to you, you have the option of taking all regional trains along the beautiful Rhine and its gorge. If you buy a Quer durchs Land ticket, you can travel and hop on and off as you like. You say "we" so there is at least 2 of you. The QdL (All Across the Country) ticket is priced at 44 EUR for the first person, and 8 EUR for each additional person, up to 5 people traveling together. It does take 4 1/2 hours to get to Aachen, but you can stop off in Cologne and see the Dom, then continue on. You just buy the ticket out of the vending machine at the airport train station, or pay an extra 2 EUR to buy from a human at the window. During weekdays, you must travel after 9 am, but even if you land at 7 am, by the time you clear immigration and get your luggage, that should be no problem. If you arrive on a weekend, it is even better, as there is no time restriction and it is a flat 44 EUR for 1 to 5 people traveling together. That is called a Schoenes Wochende (Beautiful Weekend) ticket. Remember, regional trains only. No ICE, IC, or EC trains.

This isn't an answer to any specific question, but just a side note. The stretch of track between Frankfurt Flughafen and Köln involves some of the highest speeds on the entire Deutsche Bahn network. It's a particularly impressive engineering feat because the terrain is anything but flat.

The stretch of track between Frankfurt Flughafen and Köln involves some of the highest speeds on the entire Deutsche Bahn network.

... and if you are a fan of tunnels, you will love it! ;)

Following this thread. I'm booked in Boppard for 5 nights in September and would love to take the train to Cologne then Aachen. Can i do that and back to Boppard in a day?

Judy: You'd spend 4.5-5 hours on transport using the fastest trains, 6 hours on the regional trains only, to complete that round trip. And that would leave you enough time to perhaps see Aachen, but not Cologne AND Aachen, IMHO. Better to stay over in that area for a night if you intend a somewhat complete visit for both cities.

A doable day trip to the north from Boppard is Linz/Remagen.

Linz: http://www.linz.de/pdf/Linz_Prospekt_2007.pdf

Remagen:
http://www.stadt-remagen.de/Tourismus-_-Veranstaltungen/Stadtrundgang/
http://www.bruecke-remagen.de/