Prior to almost-universal telephony, communication and control peaked via telegraphy around 1909. Of course, certain roads “went 'phone” earlier, and others later. Strictly telegraphic roads tended to be those with lower traffic upon use of telephony. That is not to say the very best roads quit telegraphy; the Nickel Plate Road, for one, used telegraphy past 1922, and then for a few more years.
Many of the big roads maintained telegraph office until the 1950s or later. One of my friends in Chattanooga retired as the last Southern Railway telegrapher sometime in the 1960s I believe. The Southern did away with train dispatching by telegraph but retained telegraph and teletype for business communications.
I don’t know exactly how universal “telephony” - whether wired or wireless - was to railroad communication and control I do know that by the early 1940s the telegraph had been almost completely superseded by the teletypewriter which dated from the early 1920s.
One probable reason that the RRs retained telegraph lines long after they had phones is cost. Today we are used to paying one flat fee for unlimited calls - not that many years ago you got x minutes per month and paid for everything else after that. “Business” phones used to be billed at a higher rate than “personal” lines. Some people are asking the same question today about “land lines”, now that almost everyone has a cell phone! My [2c]
Since in many cases the railroad owned the phone lines too, the rate wasn’t an issue. One reason the railroads kept telegraphs was there were a very efficient way of communicating. a lot of information could be transmitted very quickly. And for late 1800’s and early 1900’s it was about the fastest way of communicating.
An amusing display as on Jay Leno when they had two telegraphers race two people with cell phones to text a message. The telegraphers won hands down. And they were actually slow because the one fellow only knew American Morse and not railroad Morse, which is faster.
Dave H.