On the SP, trains with radio communication started as early as 1952. In its inception the fact was advertised on locomotives or cabooses as being "radio equipped."
But early communication was limited by distance except for communication between engine and caboose or nearby ground facility.
Since the mid 70's, again on the SP, radio contact between train dispatcher and train was commonplace. There was CTC Roseville (California) and CTC Eugene (Oregon) as the districts I worked on. But owing to the early days, there were several "dead spots" here and there. And they were AM radios.
Even into the FM range where the line of sight played a part, cuts, bowls, canyons and draws were an impediment. Today, with satellite communications, all the potential interference of the past is just that. In the past.
Using the UP as an example, any train can contact any dispatcher from anywhere on the system, with crystal clear quality.
But at first they were unwelcome amongst the road rabble.
I am speaking of "pack sets." At first found on local freight, the trainmen (Conductors, brakemen and switchmen) were worried that the advance in technology would replace men on the ground. The carriers said, "No, it'll just make it safer." And that was true for a few years. So a train crew of four, in local freight operations now consists of a Conductor and one brakeman, and one Conductor only on pool freight, that can still make switching operations as a part of their duties.
Now, the carriers are lobbying for engineer only operations. That is the most stupid **** I've ever seen on the rails. Four eyes are better than two. The engineer on the right hand side can only observe the condition of the train on right hand curves. Totally blind on the left hand side ahead and to the rear.
"Talking" track side detectors? I can name at least four derailments in my personal knowledge, including one that I was the road engineer on, that actually took the detectors with them when they occurred. The radio didn't do anything to prevent it as a burned off journal scattered my train. There was a helper engine on the rear and the engineer noticed cut ties. He called me to ask if there were cut ties ahead of me, indicating equipment on the ground. I said "no", and he said "stop the train." I started to stop and Ka-Pow, there went the air.
The derailed cars took out the detector it was supposed to guard against. Extra eyes saw it, but it was already too late.
The radio isn't a panacea as carriers would have us believe. They are handy, but they will never equal human eyes looking for problems before they begin to be the end.