How long does it take by train from Cologne to Paris?

I suppose you are landing in Dusseldorf from the US. Before you leave you can get a Eurorail pass online, but you may/may not buy a ticket direct from German rail ( I could be wrong here). You can get a ticket at the railstation in Dusseldorf when you get there. The Germain trains are great. One reason I mention that you may be able to order rail tickets I tried to find a method to get from St. Petersburg Russia to Findland when I was there and I think I could have reserved a ticket online if I had wanted to take a 14 hour train ride. If you are going soon make sure you take a coat with you to Dusseldorf it is starting to cool down this time of year and can get pretty cold.