Harold Interlocking is a large railroad junction located in New York City. It is the busiest rail junction in the United States. It serves trains on Amtrak's Northeast Corridor and the Long Island Rail Road's Main Line and Port Washington Branch, which diverge at the junction.

The Long Island Rail Road, often abbreviated as the LIRR, is a railroad in the southeastern part of the U.S. state of New York, stretching from Manhattan to the eastern tip of Suffolk County on Long Island. The railroad currently operates a public commuter rail service, with its freight operations contracted to the New York and Atlantic Railway. With an average weekday ridership of 354,800 passengers in 2016, it is the busiest commuter railroad in North America. It is also one of the world's few commuter systems that runs 24/7 year-round. It is publicly owned by the Metropolitan Transportation Authority, which refers to it as MTA Long Island Rail Road. In 2023, the system had a ridership of 75,186,900, or about 253,800 per weekday as of the fourth quarter of 2023.