The LIRR bilevels fit on the Hudson Line, as many of the coaches came in that way.
If we're going to get into running LIRR equipment off Long Island, I have to ask, is it really worth all the trouble? If the bilevels were on the latest trip to Canaan, you wouldn't be able to get out since there's no steps.
The market on Long Island is small if you're using the existing equipment (DE/DM and bilevels) as there is no real routes to use them that you couldn't just take the regular trains themselves. The price to do it is cost prohibitive, and you don't know if anyone would plunk $70.00 to ride a DE30 to Montauk (why would they?) Again, no photo stops, because there's no stairs.
While I acknowledge that all of the "old heads" at the LIRR that proposed (and operated) the fantrips are retired, we have to note that many times, their proposals were denied too for different reasons. Just because they were who they were, didn't mean it would happen.
There's a lot of planning that has to go into planning a trip, not just on Long Island, but anywhere. If you can't justify a reason for the railroad to say yes, the answer will always be no. Coming up with an unrealistic plan is also a guaranteed no as well.
Ask any of the current excursion operators today (Morristown & Erie, Bennett Levin, etc.) and they will tell you how hard it is sometimes to put a trip together. Even with all of their expertise, they are told no too sometimes.
Jay