The M9 is a class of electric multiple unit railroad cars being built by Kawasaki Heavy Industries for use on the MTA's Long Island Rail Road (LIRR) and Metro-North Railroad. They entered service September 11, 2019. These cars will replace the M3/M3A railcars built during the early 1980s, as well as expand the LIRR fleet to provide additional service after the completion of the LIRR's East Side Access project. A separate order of cars purchased for the LIRR and Metro-North using federal funding for the East Side Access project will also be designated M9A.

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.