In checking the DB Bahn site I have noticed that a single ticket from Frankfurt to Cologne is 71,00 yet a ticket from say St. Goar to Rothenburg is actually cheaper even though the distance to Cologne from Frankfurt is much shorter. Is there a reason why this is? I was hoping to take a quick side trip to Cologne on the last day of our trip.
The train from Frankfurt to Cologne is on a special high-speed track that makes this journey in just over an hour. If I recall correctly, the train will hit 300 km/hr or there about. Business people, for whom time is money, appreciate this. You can take a more leisurely and MUCH more scenic IC train that travels along the Rhine and takes 2 hours and 23 minutes for 47 euro standard price.
Yes, the full fare ticket from Frankfurt Hbf to Köln is more than the full fare ticket from Bacharach to Rothenburg, because the Köln ticket uses a more expensive high speed train. However, if you can depend on being in Frankfurt on a specific date and taking a specific train to Köln, the fare can be as low as 19€ with an advanced purchase Savings Fare.
High speed and business travelers with expense accounts. That's why the walk-up fare is so expensive.