Even the German trains are not the fastest, it won't take you 8 hours, Eleni, from Münster to Köln. Normally it is about 2 or 2 1/2, only at night perhaps a bit longer like 4 hours. I think, you can take a special ticket only for Northrhine-Westfalia which costs 21€, one way... Ok, trains are not fast, but expensive!
Would be great, Eleni, to have you here, too :)!
Have a nice week,

Yvonni