The district may lease or contract for the use of its facilities, or any portion thereof, to any operator, and may provide for subleases by such operator upon such terms and conditions as it deems in the public interest. The word “operator” as used in this section means any city or public agency or any person, firm or private corporation engaged in the transportation of passengers for hire, properly holding a Public Utilities Commission approved and authorized certificate of public convenience and necessity for the transportation of passengers.