"The girls are at the train station."
Translation:Những cô gái ở nhà ga.
No, it is not.
Basically, các is used for humans and animals, những for humans, animals and things, so they are quite interchangable.
There are still some combinations more idiomatic than others though: chào những bạn – hi guys would sound really weird, you should use các instead.
The pluralizer các is used to express that every or all elements of a group (within the context) are included, whereas những is used to say that only certain elements are included but not all. Examples:
• Các người phụ nữ việt nam. – The vietnamese women (all of them without exception).
• Những người phụ nữ việt nam. – The vietnamese women (some, not all of them).
As you can see, in both cases you have plural (women).