Complex sentence: The Shinkansen was extended as the Sanyō Shinkansen line, first to Okayama Station in 1972, and then to Hakata Station in 1975.
Split into simpler sentences: The Shinkansen was extended as the Sanyō Shinkansen line. It was extended first to Okayama Station in 1972, and then to Hakata Station in 1975.