Complex sentence: On May 29,1970, New York City and the SIRT entered an agreement for NYC to purchase the Tottenville line as part of which, the SIRT was reserved the right to operate freight service over the line.
Split into simpler sentences: On May 29,1970, New York City and the SIRT entered an agreement for NYC to purchase the Tottenville line. As part of the agreement, the SIRT was reserved the right to operate freight service over the line.