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).

The M2 cars were built by a General Electric-led consortium also including the Budd Company, Canadian Vickers, and Avco between 1972 and 1977.

Final assembly of the M2 cars using bodies constructed by Budd or Vickers was completed at GE's Transportation Division in Erie, Pennsylvania.

The M2 series replaced EMU cars dating from the early 1920s to 1954, including the Pullman 4400-series, which were originally manufactured for (and inherited from) the New York, New Haven & Hartford Railroad.

[4] As with the cousin M1 series, the M2s accompanied an overhaul of the long-neglected main line and the New Canaan Branch in which longer, high level platforms were introduced along with other infrastructure improvements.

Both the MTA and ConnDOT purchased bar cars, but complaints from riders from stations in New York, coupled with the arrival of new equipment on the Hudson and Harlem lines, led to the conversion of the ten MTA-owned bar cars to standard coaches.

The ten ConnDOT-owned bar cars, which ran on express trains to New Haven, Stamford, South Norwalk and New Canaan, remained in service during weekdays until May 2014.

The change was made due to the need to accommodate additional ridership, and because of perceived improvements the M4 model.

During the development of the order, Metro–North's operation and planning groups sought increased flexibility in the utilization of cars on the New Haven Line, and thus, decided on the triplets.

[26] M6 cars 9014 and 9015 were donated to the New England Disaster Training Center in Windsor Locks, Connecticut.

[27] M6 cars 9030 and 9031 were donated to the MTA K9 Police Training Center and Connecticut Air National Guard in Stormville, New York, near East Fishkill.