The studies that I have seen published in radio trade journals show that digital coverage will be better than analog. The only drawback is that at the very edge of reception digital will mute while analog produces only noise.

However encryption will be a very low cost option with digital radios. A railroad may decide to encrypt all digital transmissions. You will not be able to listen on your scanner. Sales of devices that will decode encryption are prohibited by Federal law.

Conversion to digital is not mandated by the FCC at this time. Narrowband is required beginning January 1, 2013. FCC type acceptance of all wide band radios expires on that date. No radios with only wide band type acceptance can transmit after that date. In previous years the FCC typically has allowed ten years for old equipment to depreciate before mandating a new standard. I would not expect digital to be required by the FCC before 2023 at the earliest.