This is not the first time the LIRR has gone down this road. In 2013, they made a similar request but instead going for 7 complete trainsets each with 2 locomotives and 2-4 coaches. However, the LIRR wasn't successful. But a couple years down the line in 2017 the MTA was able to come to an agreement with MARC (Maryland Area Regional Commuter) to lease a couple single-level passenger coaches (no locomotives). The LIRR operated 8 MARC II single-level passenger coaches on a pair of Port Jefferson branch trains during the "summer of hell".
My best guess for uses is that they'll use them on a couple Montauk branch trains during the weekdays, and then use them on the Port Jefferson or Oyster Bay branches during the weekends to tie off some additional cars for the busy Montauk Branch.
I'm very hopeful about this, I think it will help the LIRR have a slightly more successful summer then last year, I'm hoping that they will also use these on the Montauk Branch too so I can railfan them a lot! I'm going to spend a lot of time railfanning these next year!!!!