Passenger Locomotive Operations: Chicago

Can someone explain the assignment/maintenance procedure for steam locomotives assigned to the passenger fleet out of Chicago? I'm guessing that heavier roundhouse repairs were performed at 59th St. yard on the south side, after which the engines were run light to a ready track outside Union Station to await their trains. If this is the case it begs the question: Who ran the engines from the South Side to the station? Crews called for the outbound trains, or crews/hostlers specifically assigned to this duty?

Thanks for all help!