It is a current Association of American Railroads frequency coordination recommendation to include NXDN on all frequencies to be available for future use. So it is showing up on most licenses but it does not mean it will be implemented soon. Also, most current major railroad radio purchases are NXDN capable and often programmed with both the AAR analog and NXDN channels.
I don't know about LIRR, but many railroads are initially using NXDN only for localized operations in a yard and also for Railroad Police where it is often encrypted. A switch to NXDN for road and dispatch use would require that all units, including "run-through power", be NXDN capable but that probably would not be an issue for LIRR as it is for freight Class 1s.
So when NXDN shows up on a license it does not necessarily mean a switch in imminent or there is any mandate to switch. It is just preparation for the future in most cases.
It is a similar situation to P25 being added to many Public Safety licenses although they may remain analog for now.