There seems to be some confusion about federal regulations and their impact on what equipment is necessary for passenger car operation.
As far as the FRA, or any other regulatory body for that matter, is concerned, the LIRR can use any motive power it wants to pull its passenger cars. The LI 1500 engines can pull C-3's just as M-1's can pull C-3's if the Railroad chose to do that.
Of course, door lights, intercoms, and buzzers would not work. There is no regulation that requires these. While the Railroad frowns on the use of radios for this type of communications, hand signs are actually considered a superior device for passing information through a consist. Many of the better crews use handsigns almost exclusively, because buzzers can fail and provide wrong indications.
Since the LIRR has rules governing how a train would be operated without a door-closed indicating light, the FRA may require that the crew follow those rules, where it can, but a 1500 can easily operate in regular service with C-3 equipment.
No reason why it cannot. It has, it does, at times, and it shall continue to when needed.
Just to set the record straight, while I perhaps posess some specific knowlege, it is about mostly insignificant things, and my limited wisdom serves only to remind me of my foolishness in times past..
Clem