Your personal cellular phone is confined by technologies and marketing restrictions to where it can get service. Railfone, roughly speaking, looks around for any available (analog) cellular base station as it tries to establish a connection for you. In fact, even though Railfone has a significant player in Verizon, it seldom uses Verizon infrastructure, especially in rural settings. It is, therefore, more likely you can get Railfone service than personal cellular service, even though both use the existing cellular infrastructure. This is particularly the case for digital cellular subscribers who often can't get service in rural locations.
Amtrak originally got involved with the system back in 1988 for a host of reasons. One was to encourage the cellular operators to install cellular radio repeaters along lonely sections of track so that the operating staff could enjoy fewer dead spots in their cellular coverage. There was only minimal activity on this issue. The whole situation has changed with various incompatible digital cellular installations in the 1990s, and will change more as the analog infrastructure Railfone uses starts to disappear during the present decade. The legal requirement for analog channels on all 850 MHz cellular base stations was recently lifted.
There is some talk of offering phone service via the next-generation (digital) private radio systems some railroads are installing for their own use, but I would not hold my breath. Thanks to the ADA (Americans with Disabilities Act) Railfone may outlive the already fading Airfone system, which is already turned off on some airlines.