The Staten Island Railway rapid transit system solely serves Staten Island, operating 24 hours a day. The Port Authority Trans-Hudson (PATH train) links Midtown and Lower Manhattan to northeastern New Jersey, primarily Hoboken, Jersey City, and Newark. Like the New York City Subway, the PATH operates 24 hours a day; meaning three of the six rapid transit systems in the world which operate on 24-hour schedules are wholly or partly in New York (the others are a portion of the Chicago 'L', the PATCO Speedline serving Philadelphia, and the Copenhagen Metro).
What 24-hour rapid transit system is outside the United States?
Copenhagen Metro