One of the first items on the list was a massive order of brand new self-propelled electric railcars that could replace the remaining MP54s and provide modern levels of comfort and performance.

The cars were fully air conditioned, accommodated only high-level boarding, used lightweight construction, and were built with a top speed of 100 mph (161 km/h)[5] and support for Automatic Train Operation.

The Metropolitan cars were also the catalyst of change for their respective systems as the high-level boarding required all stations in the electrified zone to be rebuilt between 1966 and 1968,[6] and the increased power demand forced the LIRR to update its third rail power supply from 650 V DC to 750 V DC to take advantage of the car's performance.

The Metropolitans blurred the line between traditional commuter rail and rapid transit, with the later R44 and R46 series of cars for the sister New York City Subway adopting many of their design elements.

Compared to the older cars with their drop sash windows, slow speeds, rough suspension, and growling gearboxes, the Metropolitans ushered in a new era of commuting in the New York region.

[dubious – discuss] The M1 series were funded by both New York State and the then-fledgling Metropolitan Transportation Authority, which gained operation of the lines partway through the order.

[4] The 770 M1s (9001–9770) built for the LIRR between 1968 and 1973 represented the largest single order of electric multiple units in North America up until that time.

In addition, 178 M1As (8200–8377) were built for the former New York Central commuter operations from 1971 and 1973, allowing the railroad to replace its remaining pre-war MU cars and the 100 4500 series ACMUs (which were in need of a 20-year overhaul).

Essentially compatible with, and (on the exterior) very similar to, the M1 series, the M3s had updated mechanical elements, such as the use of General Steel GSI 70 trucks and a few other small differences.

In preparation for the retirement of the M1s, the Sunrise Trail chapter of the National Railway Historical Society hosted a "Farewell to the M1s" fan trip on November 4, 2006.

[20][21] Metro-North also planned to overhaul their M3A units,[22] but later stated that they "are working with LIRR to procure new M9A cars in the next Capital Program to provide additional capacity and replace M3s.