Complex sentence: The New York City Subway, which has become one of the world's largest, opened its first section in 1904, a fully independent four-track line stretching 9 miles (14.5 km) from City Hall to 145th Street.
Split into simpler sentences: The New York City Subway, which has become one of the world's largest, opened its first section in 1904. This section is a fully independent four-track line stretching 9 miles (14.5 km) from City Hall to 145th Street.