Kik vannak is plural, so it should be Who are and not Who is.
In theory yes, but in practice, we use "Who is [verb]ing" in English even if the expected answer is two or more people.
Similarly with "What is [verb]ing" even if the expected answer is two or more objects.
The problem is that there is no -ing form. The hungatian sentence means "who are [the people] behind the old railway station?"
We are learning Hungarian not English. Who are/is should both be accepted