Cologne is in the northwest of Germany, easily accessible by train from three major German airports and only a few hours on the train from Brussels, Amsterdam and Paris. Check out the airports’ web sites for more information.
Trains are frequent and easy to use. The high speed train from Frankfurt Airport for example takes only an hour to get to Cologne main train station, you could never drive the over 200 km in that time! For visitors from abroad there are rail passes that may be attractive if you intend to travel more than just to Cologne for the conference. Follow this link to the German Rail web site.