Travel from Paris to Kassel

"Which rail line"?????????????????? Maybe you mean which route. Each country has their own rail company, so in Germany you will travel with German Rail, the Bahn. From Paris to Germany, you might travel on one of the high speed trains, TGV of French Rail, or ICE of German Rail, or you might travel on Thalys from Paris to Köln. That line is owned by French and German Rail, faster and more expensive than trains of the national rail lines. However, if you book online from the German Rail website you can probably get the best fare, Europa-Spezial Frankreich, for as low as €39. There's a TGV/ICE 5½ hr connection leaving Paris at 9:09, changing in Frankfurt, and getting to Kassel at 14:41.