Train from Cologne to Frankfurt

Advance buying of Intercity trains (IC or ICE) comes with huge discounts. Those discounted tickets are only valid for that specific train.
The simplest way is to go to www.bahn.de/p_en/view/index.shtml. Payment is by credit card and tickets are sent as a pdf for printing at home. On the train you can be asked to show the credit card you used (but you can also indicate another credit card as a means of verification).
I understand that tickets for 10th December onwards are available from 18th October.

"We need tickets from Cologne to Frankfurt on Dec.14 - do we need to get those tickets now? If so, how?"

Normally, advance-purchase is best. Will you be spending the night before in Cologne? Or are you arriving at Cologne/Bonn or Duesseldorf airport on the morning of Dec. 14.? That makes a big difference.

You can't buy them now - I think your travel date will be available online at the DB website in about 10 days or so. You can check a sample date through Dec. 10 to see how the trains run. The Dec. 14 schedule will probably be similar. The fastest route is quick but dull. The Rhine River route takes longer but is very scenic. Which one interests you?

do we need to get those tickets now?

No. But you will pay the lowest price by buying as soon as they become available for that date. Just note, though, that this locks you into taking a specific train.

If so, how?

As noted above.

Or can we buy them at the train station that morning?

Yes, but you will pay the full fare price.

The fastest route is quick but dull.

I disagree. The route through the Mittelrheintal may offer more scenery, but the ICE from Köln to Frankfurt passes through the attractive towns of Siegburg, Limburg and Montabaur. Plus, the tracks themselves are an engineering marvel. The trains achieve the most blindingly fast speeds on the entire rail network, despite cutting through terrain that is anything but flat.

"Normally, advance-purchase is best. Will you be spending the night before in Cologne? Or are you arriving at Cologne/Bonn or Duesseldorf airport on the morning of Dec. 14.? That makes a big difference."

Russ - we will be completing a Rhine Xmas Market River Cruise. Spending 4 nights in Cologne, then heading to Frankfurt for another 4 nights. Why does this make a big difference on when/how we purchase the tickets?

And - will we know which is the boring route VS the scenic river route? Tom's comment that he prefers the other one has caught my interest. Will we know which route we are booking?

Thanks, Terri

"Why does this make a big difference on when/how we purchase the tickets?"

When you're flying into a city and then taking a connecting train right away, you can't predict how long the entry formalities of Immigration and Customs will take. For that reason, it's usually recommended that you not try to buy advance tickets in that situation.

Since you will already have been in town for a few days, there's no problem committing yourself to a specific train departure. Therefore, you can take advantage of the advance purchase discounts without worry.

"And - will we know which is the boring route VS the scenic river route? Tom's comment that he prefers the other one has caught my interest. Will we know which route we are booking? " That's something you can determine yourself. at the DB itinerary page: http://reiseauskunft.bahn.de/bin/query.exe/en

If you want to use the high-speed route Tom likes, after your results come up, just click on "show details" to see what towns your train stops in. If it's non-stop, you're on the right route. If it stops in Siegburg, Limburg, or Montabaur, you're on the right route. However, if it stops in Koblenz, you are on the Rhine River route instead.

But if you want to go along the Rhine, you must do an extra step in order to see ALL the trains that go that way. Click on "add intermediate stops" where it says "Stopover" - then enter Koblenz Hbf. Do not enter any stopover time. The search results will show only trains that travel the Rhine between Frankfurt and Cologne.

I traveled the high speed route from Frankfurt to Cologne a couple of months ago and found the ride fast but scenically pretty dull and uneventful. I've been in Limburg - a very nice town. But if you don't stop to look around the town, I don't see the fascination with making a train stop there. (That said, YOU will have already seen the Rhine on your cruise.)

For other readers of this thread: The Middle Rhine valley route is a UNESCO World Heritage site and by most people's standards the most scenic railway segment in all of Germany. Between Bingen and Koblenz you will pass by about 40 different castles, most of which you can get a pretty good view of from the train. Many people cruise this river segment, usually in the warmer months, to get a better look. In December, the green grape vines in this wine region will have died off, but the steep cliffs and river scenery IMO is head and shoulders above what you'll glimpse out the window along the high-speed route.

Scene near Bingen
Rheinstein Castle
Boppard