Complex sentence: The only grade crossing of the subway system, where East 105th Street formerly crossed the Canarsie Line, was eliminated in the 1970s and is located at the site of the current station house.
Split into simpler sentences: The only grade crossing of the subway system was located at where East 105th Street crossed the Canarsie Line. It was eliminated in the 1970s and is located at the site of the current station house.