Found this online:The service is due to go live at 60 additional subway stations by the end of this month, bringing the total to close to 150.

The full rollout to all 472 subway stations and all MTA bus services is scheduled for completion by the end of 2020.

Omny will then begin to support existing MetroCard affiliates, including Path and AirTrain, in 2021 and 2022.

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Since AirTran is $5.00 and an "affiliate", this is probably why it's later. It's easy to make sure the system is constantly charging $2.75 every time, without having to track a small subset of the system. I'm sure they want the main system to work, before adding the one-offs (Path, Airtrain, LIRR)

(to also respond to the LIRR Machines, Contactless works on all machines for Apple, Google and Samsung Pay, I do Samsung Pay all the time with mostly no issue, but I have found that some machines are stubborn related to the phones, they do work, but not as consistently as the credit card tapping)