I guess you are talking about a night train? So then there wont be another one that can reserve your bed from Harburg to Hamburg Hbf. I would just talk to the waggon stewart to be safe. If he sees it as a problem (I really dont think so!), you can just get out in Harburg and take a S Bahn to Hamburg Hbf.

Hey,

don’t worry, there won’t be a problem. You can get off at any station in Hamburg as it is covered with your Interrail pass and also the reservation price doesn’t change due to going to another station… The conductor might not even notice where you get off.