Subway station

[HollowCullen]

Hi! You`ll see, i`ve been recently entering into the world of BAT and right now i`m doing an underground station found in my city and the results are still good, but as any other underground station, it has to have it`s underground stage, but i don`t know how to do it.  It was told to me that i had to put the main building in the underground part or something like that but what i want is that when entering the underground menu in the game the underground part appears, that supossedly i made how could i do it?
i leave you a screenshot.

       
how can I do this?

   
or this?

Thanks a lot