Some railroads are buying Icom NXDN radios, some Kenwood, and some Ritron, and some railroads are buying all 3. The Ritron doesnt even have trunking, and the Icom and Kenwood trunking does not work together.
So If trunking were to get used in the yards, then only yard engines with a compatible NXDN trunking radio could talk on those talk-groups. That would be an interoperability issue, as some railroad use engines from other railroads in their yard switching duties.
Railroads ARE going NXDN digital sometime in the not so far future, and road operations will be Conventional and in the clear. They dont need to add any base stations, or do anything to the lineside infastructure, because digital carries just as far as analog (I know because of extensive testing with NXDN radios). Many of the railroads around here have already converted the wayside base radios to NXDN, so all it will take is a "flip of the switch". Also, out in the rural areas, like here in Southern Illinois, most if not all main line road channels will stay on the same frequency they are on now, just transmitting NXDN mode.
Mainly the railfans in the metro areas of big cities like Chicago and LA are going to be the ones have to adjust to a new frequency.
I have an Icom F3161ds, and it and my F6121d uhf digital mobile are my favorite Icom LMR radios. Our ham group is using NXDN, the local PD and EMA are using NXDN, there is a trucking company that has a wide area NXDN system.
Most of the class 1 railroads in this area have already added 4k00 emmission to the licenses.
Im ready and waiting!
Steven