The problem with any pet project like this is the very limited results you're going to get. There isn't unlimited money to electrify, double or triple track, for new equipment, et cetera. Besides, where are you going to fly to? The only places you're going to get enough passengers for a line-haul is Florida, and with nearly hourly service on a variety of carriers on closer airports, why bear the extra time or expense of going NORTH to go SOUTH? They're re-working the airspace in the northeast, so I don't necessarily buy the argument that Islip or Westchester service can't be expanded. You'd be more limited to the number of gates, but they're never close to being all full.
If that all-important rail link be needed to Islip, have Southwest issue through tickets from NYP to Ronkonkoma, and have a shuttle bus from the train to the airport. The great untapped air market is in the other direction…Trenton Mercer. It's right on the West Trenton SEPTA R3/NJ Transit proposed West Trenton line. Again, some shrewd marketing, somewhat frequent service, and a few shuttle buses, and you're in business. No need to ask if you build it, will they come?