The FCC does NOT require digital transmission at this time but probably will at some time in the future. Digital transmission allows more signals to operate in the same spectrum space.

In the past the FCC has given ten years advance notice for a major change like requiring all stations to use digital equipment. This will allow radio owners to fully depreciate their old equipment. So a railroad that converts to digital today probably is looking toward the future when digital will be required. If I had to replace old worn out analog radios today, I probably would replace them with digital.

If the digital signals are encrypted, you're out of luck. You'll never see a scanner that can decode encrypted signals because they are prohibited by Federal law.