Complex sentence: The Staten Island Tunnel is an abandoned, incomplete subway tunnel that was intended to connect railways on Staten Island (precursors to the modern-day Staten Island Railway) to the BMT Fourth Avenue Line of the New York City Subway, in Brooklyn.
Split into simpler sentences: The Staten Island Tunnel is an abandoned, incomplete railway/subway tunnel in New York City. It was intended to connect railways on Staten Island (precursors to the modern-day Staten Island Railway) to the BMT Fourth Avenue Line of the New York City Subway, in Brooklyn.