It seems unsafe in this day and age to have a train with hundreds of lives totally out of communications in the event of trouble issues of many kinds--be it medical, crime, breakdown, or whatever. If there are spots with comm. 'dark territory', they need to be addressed. The building of cell tower, radio repeater, or even old lineside RR telephone boxes should be established.
If all else fails, locomotives should be provided with Iridium satellite phones, that do not depend on landbased cell towers.
In railroad terms, all "dark" means is a line is not signaled, just controlled by dispatchers
by radio. As AlanB pointed, the RR's radio is available everywhere along the line. If it there is an area of bad reception due to geography, the railroad puts in repeaters, and they run antenna lines in the tunnels. If the conductor's handset isn't powerful enough in a certain area, he can radio to the engineer and the engineer's more powerful transmitter in the cab will work.
Chill, dude. And don't get your facts from a bad movie ("Dark Territory").