Detailed Instructions: Given a travel related situation, and given a specific question about the situation, summarize the question into a title
Problem:I am buying a train ticket from Paris to Dusseldorf (Thalys). Can I hop off in Koln 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.

Solution:
When going by a train from Paris to Dusseldorf (Thalys), can I hop off in Koln and then hop on again?