It's pretty safe and would suggest it also to single/female travellers. It's also relevant which day of the week you arrive. It can be pretty lonely/calm on the train station and later in the train (just some other tourists, still recommended to take the train) during the week but it will get crowded (in a positive way in that direction) the closer you get to the city center. Depending on where you go to, starting with Thursday it's crowded anyway but you will have some party folks having already/still a beer or two. Only much later (2 to 5 o'clock) in only a few suburbs you might feel uncomfortable. So it rather depends WHERE you are going.