Really nothing too exciting. When I lived in Germany I often took the train to get to various places and I frequently ended up taking the last train of the day, or taking a local train/bus when meeting friends/going out which stop at/near the train station; so I often ended up being at/near various train stations between 12a.m.-2a.m.

Basically plenty of people from the groups mentioned above. Usually the main hall and the area between the main hall and where the taxis wait is safe whereas the side streets and back/side entrances of the train station attract various people I prefer not to interact with:

- prostitutes
- pimps screaming at prostitutes
- drug dealers (it's not uncommon to be actively approached whether you want to buy something if you're under 30)
- drug addicts and homeless people
- groups of Arabs/Turks harassing people, openly playing around with their knives
- gypsies aggressively begging

Except for some policemen attending to a man who had just been stabbed (not fatally) I fortunately never experienced any incidents first-hand, but in many cities the area around the train station isn't necessarily a place you want to hang out at night.