Given a travel related situation, and given a specific question about the situation, summarize the question into a title

I am buying a train ticket from Paris to Düsseldorf (Thalys). Can I hop off in Köln to spend sometime and then hop on an RE (regional train) on the same day (within half of a day)? Or do I have to buy a separate tickets? I know I cannot board the Thalys again, because it requires a seat reservation.

When going by a train from Paris to Düsseldorf (Thalys), can I hop off in Köln and then hop on again?