Which safe house and hiding places?
The known ones. The ones known to the conductors. [adjective + prepositional phrase = adjectival phrase]
It can also be read as "the houses and places which were known to the conductors." if that helps you.
Here are some more examples:
The prizes given to the students were expensive.
The sentence handed down by the judge seemed excessive.
In each case, the phrase qualifies the subject noun, so it's adjectival.
Student or Learner