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.

Long Island is an island in southeastern New York state, constituting a significant share of the New York metropolitan area in both population and land area. The island extends from New York Harbor 118 miles (190 km) eastward into the North Atlantic Ocean with a maximum north–south width of 23 miles (37 km). With a land area of 1,401 square miles (3,630 km2), it is the largest island in the contiguous United States.