Complex sentence: As the first line neared completion, New York City offered it, "pro forma", for private operation, knowing that no operator would meet its terms, so the city declared that it would operate it itself, formalizing a foregone conclusion.
Split into simpler sentences: As the first line neared completion, New York City offered it for private operation as a formality, knowing that no operator would meet its terms. Thus the city declared that it would operate it itself, formalizing a foregone conclusion.