M2

The M2 "Cosmopolitan" series (numbers 8400-8849) 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 and Hartford Railroad. 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. The first M2s were accepted for service on April 16, 1973. 144 base order cars were built in 1972-1974, followed by a 100-car option in 1975. These cars had been scheduled for delivery in spring 1971, but were delayed due to technical problems. Aside from the technical differences of the New Haven Line (electrification via overhead catenary instead of third rail), the cars are similar to the sister M1A order and, in times of equipment shortages or severe weather, the M2s have run on the Hudson and Harlem lines. Most of the other differences are in the interior and exterior appearance of the cars, such as red striping on the exterior rather than blue, the interior wallpaper having both the New York and Connecticut state seals and the obvious pantograph and mechanical apparatus on the roof. Both the MTA and ConnDOT purchased bar cars, but complaints from riders from stations in New York, coupled with 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. 48 M2s were overhauled starting in 1994, with 24 returning to service in 1995.

M4

After the LIRR and Hudson/Harlem lines received an updated version of the original Metropolitan series of cars in 1984-86 (the LIRR M3 and Metro-North M3A series), plans were announced for a similar undertaking on the New Haven Line. It sought additional cars to increase service on the line after projections indicated an increase in ridership. Metro-North originally planned to purchase 44 additional M2s, but decided to award a contract for 54 M4 cars (8900-series) in 1987-1988 to Tokyu Car. The change was made due to the need to accommodate additional ridership, and because of perceived improvements the M4 model. The MTA Board approved the $77.3 million contract on December 20, 1984. These cars were built in the Brooklyn Navy Yard. Nemko assembled and finished the cars for Mitsui, which shipped the parts made by Tokyu. In October 1986, these cars had been expected to enter service the following year. The first cars were expected to arrive in early 1987. Identical to the M2s, Tokyu Car initially gave the "Triplex" brand name to the M4 cars to highlight their being a three-car set, as opposed to the married pairs of the M2s. These cars came to be known as "triplets" by railroad personnel. 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. The order was designed to ease maintenance and to improve reliability. The cars used fabricated bogies with air suspension, unlike previous cars in the M series.