But people who would use the service most likely aren't going to be your every commuters into the city and back but people traveling beyond.
Amtrak's high-speed visioning document at one time showed going out Long island and then crossing the sound by tunnel to get to New Haven.
I think LI NIMBYs killed it.
But it is instructive in at least station-spacing and where the demand-area was thought to be:
"Ronkonkoma" (Islip Airport & the "Suffolk County" "outer Long Island station
"Nassau Hub" which would be someplace between Mineola and Hicksville (The "Nassau County" central Long Island station)
For me the only question is whether Queens is big enough to get its own stop (I say "yes")
A Jamaica Amtrak stop would be killer: EJZ Subway, most LIRR, + JFK Airtrain