[6] 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.

[7][8] In mid-2012, the MTA issued a joint procurement request for the LIRR and Metro-North for a total of up to 676 M9 railcars, set for delivery between 2016 and 2020.

[8] In June 2018, Metro-North announced that they would elect to not exercise their options for the M9 order, instead overhauling their M3A units to extend their lifespan.

[11] However, that November, Metro-North stated that they "are working with LIRR to procure new M9A cars in the next Capital Program to provide additional capacity and to replace their existing M3s instead of overhauling same.

"[12] By October 2017, car production was delayed again as the 92-car base order was now scheduled for delivery between July 2018 and March 2019, at a cost of $393 million.

[13] Construction delays in the East Side Access project meant that delivery of M9 and M9A railcars intended for use on that line had been deferred.

[7] The production of the base order had suffered delays, and the first eight-car pilot train was now scheduled for testing at the Transportation Technology Center (TTC) facility in Pueblo, Colorado from June to August 2017, with the first fourteen-car pilot train on the LIRR was scheduled for testing from October to December 2017.

The original timeline had been delayed by 13 months due to defects found in on-site testing, as well as the onset of the COVID-19 pandemic in 2020.

[23] As a result, some riders began colloquially referring to the M9 cars as "unicorns", since they comprised less than 5% of the LIRR's fleet as late as 2022.