kvar chatzót ve-hem adáyin ratzím.
in modern hebrew could חצות be noon too?
yes. "חצות היום" = midday/noon. "חצות הלילה" = midnight. noon is also called "צהרים" which is the more common word.
Thanks, because when you see the translation of חצות only says midnight, i must be completed?
Not exactly, the use of חצות as noon is not modern, can be found in old texts. (yes, I might have argued the opposite elsewhere, but it's really not common in Hebrew today, besides religious contexts)
Is this something one might say about busses or trains (הם רצים)?
I can't help but see it as incomplete without the "it is". Is there some way to make it clear why starting directly with כבר, without saying "it is" before it, is sufficient for the meaning ?
As the word "is" doesn't exist in Hebrew - no. כבר means "already", regardless of the "it is" part
I entered "it is already midnight and they still are running", and it was marked wrong