Make sure to call ahead and ask if the lower-level "Schaudepot" is open on the day you visit. Half of the cars are in there (less than 50 in total), and it's sometimes closed for rearrangement. Like it was for me, last week.
Oh, and when you're there make sure to visit the Maritime Museum just round the corner. I have zero interest in anything maritime (or so I thought), but that is perhaps the greatest museum I've visited (3 times so far).
Another oh: Hamburg's (arguably Germany's) biggest tourist attraction, the Miniatur Wunderland, is also round the next corner... : )