What is the shortest distance Airport to Cologne Germany From San Francisco

The most commonly used airports for such a trip are:

• Frankfurt International Airport (FRA)
• Dusseldorf International Airport (DUS)

Both are served by US-American Airlines from the US. There are also non-stop flights to Frankfurt, which you may want to prefer.

Airport Cologne-Bonn (CGN) is actually closer, but less commonly used for such trips. Trying to use it for a transatlantic trip typically leads to higher airfares in my experience.

One option you can consider is that Lufthansa has a partnership with Deutsche Bahn, the German train operator. You can go to Lufthansa.com and make a single booking consisting of a flight from San Francisco to Frankfurt (operated by either United or Lufthansa) and a train ticket from Frankfurt to Cologne, with a guaranteed connection in case your flight is delayed.

In this example, "flight" LH3612 is actually a train from the airport to Cologne, but your air ticket is valid for this train:

You'll have to research whether this is cost-effective based on your travel dates. For some random dates I tried next February, the combined air+rail journey to Cologne was $10 cheaper than just the flight to Frankfurt alone, so there could be some savings booking together.

There's also a separate program called Rail&Fly. With this, you book a flight to Frankfurt (or elsewhere in Germany) and then click the "Buy train tickets" button to purchase deeply discounted train tickets to/from any station in Germany valid for the dates on or near your flight dates.

You can check all of these options to see which is cheapest and/or more convenient for your travel dates.

Frankfurt Airport has a long distance train station attached to the airport, with a direct link to Cologne. The train Frankfurt - Cologne is pretty much the fastest train run in the country. It is one of the few places in Germany where the ICE actually runs at 300km/h (I have a picture to prove it!), and there is only 1 stop in between, the trip takes about an hour.

Munich is 3 hours away from Cologne, at least, and you are still 45 minute away from Munich station, the airport is quite a distance out of the city in the middle of nowhere.

You can check the train times on www.bahn.de

Alternatives for Frankfurt, which has the best connection to Cologne, are Amsterdam (AMS, Schiphol airport, the Netherlands) and Paris, (CDG, Charles de Gaulle, France) and likely a couple more I did not check.
The travel time from the USA to Europe will be about the same, maybe a bit less, but not a lot.

Both cities I mentioned have direct trains to Cologne and train stations right at the airport, but neither airport seem to have those direct trains, you will have to change between trains if you fly there. So while it may be a good alternative if you have trouble finding well priced tickets to Germany, it will not be better than a good second place.

I used the Bahn.de site for the train information, they will also sell German and international train tickets online.