Complex sentence: In the past, the 1 train operated as a skip-stop service in tandem with the, which was discontinued after May 27,2005; from 1994 onward, this skip-stop separation existed only in Upper Manhattan during rush hours.
Split into simpler sentences: In the past, the 1 train operated as a skip-stop service in tandem with the. This service was discontinued after May 27,2005; from 1994 onward, this skip-stop separation existed only in Upper Manhattan during rush hours.