The M2, M4 and M6 were three similar series of electric multiple unit rail cars produced by the Budd Company (M2), Tokyu Car Corporation (M4), and Morrison-Knudsen (M6) for the Metropolitan Transportation Authority (MTA) and the Connecticut Department of Transportation (ConnDOT). Initially branded as the Cosmopolitans, the cars were later more popularly known under their model names, M2 (1970s cars), M4 (1980s cars), M6 (1990s cars). They ran on the New Haven Line (then part of Penn Central, now a part of Metro-North) for most of their service life.

The M4s and M6s were retired by Metro-North in 2015, followed by the last M2s in 2018. They have been largely replaced by new M8 railcars. One pair of M2s has been preserved at the Danbury Railway Museum in Connecticut.