Having been around at/near various train stations past midnight many, many times (Frankfurt, Hamburg, Dortmund, Essen, Duisburg, Bochum etc.) I've certainly seen a whole lot of unpleasant people (and very few policemen/security staff), but I agree the train station should still be quite busy around 10p.m. I didn't mean to scare anyone off, but the areas around German train stations aren't the most pleasant or safest places to hang out past midnight IMO.